Laboratory experiments were conducted to screen plants i.e. Celosia argentea, Celosia cristata, Mirabilis jalapa, Mangifera indica, Moringa oleifera, Ziziphus jujuba, Ziziphus mauritiana, Delonix regia, Morus alba, Albizia procera and Albizia lebbek, for their allelopathic potency on Lepidium sativum. Maximum inhibition (97%) to root elongation was observed at 10% concentration of Celosia argentea extract; however, Mangifera indica at 10% concentration totally inhibited the germination of seeds. On the basis of results of present studies and literature survey, Celosia argentea was selected for further study. Different concentrations of Celosia argentea parts i.e. roots, stem, leaves and flowers were compared for their; (a) allelopathic potential (b) total phenolic contents and (c) individual phenolic compounds through Mass Spectrometry (MS). Based on literature survey and present studies, leaves of Celosia argentea were selected for isolation, purification and identification of herbicidal compounds using bioassay guided isolation method. Solvent extraction method using different solvents and High Performance Liquid Chromatographic (HPLC) method using different solvent systems were optimized. Finally, two types of extraction methods i.e. aqueous and methanolic extractions were used. Aqueous and methanol extracts were partitioned with n-hexane, then with ethyl acetate and the remaining extracts were divided into two parts. One part of each extract was subjected to acid hydrolysis to release the phenolic compounds from sugar moieties and then extracted with ethyl acetate, while second part was lyophilized. All the organic solvents were evaporated and residues were dissolved in methanol to prepare 4000 ppm solutions and were used for bioassay study against Lepidium sativum. The highest inhibition of seed germination (100%) was caused by acid hydrolyzed and ethyl acetate fractions followed by lyophilized fraction. Fractions with inhibitory potency i.e. ethyl acetate, acid hydrolyzed and lyophilized fractions were used for analysis through Thin Layer Chromatography (TLC), High Performance Liquid Chromatography (HPLC) and Liquid Chromatography Mass Spectrometry (LCMS). More than 60 compounds including quercetin, quercetin 3-O-glucoside, quercetin pentose glucuronide, gallic acid, gallic acid 4- O-glucoside, m-hydroxybenzaldehyde, pholoroglucinol, phloroglucinol glucoside, mcoumaric acid, catechol etc. were identified as allelopathic agents. Most inhibitory fractions i.e. ethyl acetate and acid hydrolyzed fractions of methanolic extract were further divided into five fractions through Preparative Thin Layer Chromatography. Different concentrations of five fractions that were obtained from this separation were subjected to bioassay study against Lepidium sativum. The active fractions i.e. F2 and F5 were further separated on Preparative Thin Layer Chromatography in order to purify individual compounds. Active compounds were then analyzed with LCMS and Nuclear Magnetic Resonance (NMR). To our knowledge compounds such as 3,5-dihydroxybenzaldehyde, p-coumaric acid were first time identified as herbicidal compounds in Celosia argentea. In addition to this work, Mirabilis jalapa, Mangifera indica, Delonix regia, Morus alba and Moringa oleifera leaves were also subjected to bioassay guided isolation method and active fractions were analyzed for allelopathic compounds using HPLC and MS.