The present research was aimed to study the effects of the largest oil spill incident ever recorded in the history of Pakistan on the species composition of centric diatom. Evaluation of short term effect of oil spill on species assemblages of centric diatoms was made from samples collected just after Tasman Spirit Oil Spill (i.e. pre north-east monsoonal period or autumn season), 13th August 2003 and compared with long term effect after one and a half year of spill (i.e. post north-east monsoonal period or spring season) from samples collected under the Bio-remedial project. Five sampling stations were selected under the project of Tasman Spirit Oil Spill and 16 stations were identified under the Bio-remedial project considering polluted and non polluted area of Clifton beach. Hydrographic parameters including water temperature, salinity and pH were also recorded from each station. The samples (water) were cleaned following a specific protocol to study the morphology of centric diatom in laboratory under the light microscope. Scanning Electron Microscope was another method used to study the detailed morphological structures and specific protocol was followed to get best results. All possible morphometric measurements were noted while taking images. Identification of centric diatom specimens’ up to species level was done by using local and international flora (literature). In this study a total of 40 species belonging to 15 genera of centric diatoms were recorded among which only 10 species were estimated exclusively from samples of Tasman Spirit Oil Spill, whereas 24 species were isolated strictly from Bio-remedial Project samples. All other species were identified in samples collected from both sampling sites. Rhizosolenia and Chaetoceros were the most diverse genera with 10 and 8 taxa respectively. The subsequent diverse genera were Coscinodiscus and Odontella with 5 & 4 species respectively. Moreover, rest of the genera represented comparatively lower species diversity. It is noted that Coscinodiscus asteromphalus, Chaetoceros pseudocurvisetus, Ch. pseudosymmetricus and Bellarochea malleus were the first record from the coastal water of Pakistan. A great number of species recorded in this study were neritic whereas few were oceanic and oceanic-neritic. It was also noted that dominant centric diatoms were tropical few centric diatom species were of temperate and cold water origin, as the sampling season was pre and post north-east monsoonal period when mixing of water is not at its peak from the adjacent water as it is deeper colder water regime. A marked difference in the distribution of centric diatom species was observed between Tasman Spirit Oil Spill and Bio-remedial Project samples. Among 40 species only 17 were isolated from Tasman Spirit Oil Spill sampling sites. This discrepancy may be due to inhibition in the growth of phytoplankton assemblages as a result of deleterious effects of crude oil. Increase in number of taxa isolated from Bio-remedial Project samples indicates the gradual recovery of ecological conditions after a period of 1.5 years supporting the growth of phytoplankton. There were some species including members of genera Odontella, Rhizosolenia and Guinardia recorded with unusual or deformed structural shape which may indicate the effects of oil spill on the physiology of the species. Species diversity, richness and equitability of centric diatoms were calculated by using ‘diver’ program in which Shannon-Weiner, Margalef and Pielou formulae were expressed respectively. Species diversity, equitability and richness were highest in post monsoon period or spring (BP) and lowest in pre monsoon period or autumn (TSOS) which at one hand indicate the inhibition of photosynthesis, primary production and respiration mechanism due to the presence of high concentration of crude oil in the area. It also indicates that such disasters inhibit cell growth, reduce cell number, bio volume and minimize the ability to utilize nutrients. Cell counts of species occurring at each station were also taken and data was analyzed by using cluster analysis technique. The results accumulated from cluster analysis showed two distinct clusters in the sampling stations of Tasman Spirit Oil Spill. Group-1 comprised 4 stations and that of group-2 only 1 station. Among 4 stations of group-1 Odontella mobiliensis was dominant with mean value of 19 whereas, Odontella sinensis was dominant in group-2 with 95 cell counts per ml. The lowest density of Coscinodiscus radiatus and C. asteromphalus with mean value of 1.25 and 3 cells per ml was observed in group-1 and 2 respectively. On the contrary there were 2 major groups obtained in samples of Bio-remedial Project which were further divided into sub groups. The dominant species in major group-1 was Rhizosolenia imbricata with mean value of 8.33. Moreover, Coscinodiscus radiatus and C. wailesii showed dominance in group-2 with mean value of 1.1. In group-1 and 2 mean values with lowest density of Hemidiscus kanayanus (0.16) and Rhizosolenia hyalina (0.1) were observed respectively.