The main objective of this research study is to explore students conceptions and to promote conceptual change through application of constructivist approach. This dissertation will develop an awareness of the existence of high frequencies of alternative conceptions in science students at secondary level. The population of the study was secondary schools boys and girls (classes 9 and 10) of government schools of Lahore city. The research study accomplished in three phases. In phase-I, the sample of the study was consisted of 120 subjects of class 10 randomly selected to explore students understanding as an evidence of high frequencies of alternative conceptions to conduct experimental research study in phase-III. The phase-II was the establishment of constructivist teaching strategy for the experimental treatment to the subjects in phase-III. The phase-III was consisted of randomly selected sample of 440 male and female students of class 9 from four public schools. Experimental treatment and measure of conceptual change was carried out. Four basic concepts of chemistry were selected, such as composition of matter, solutions, chemical bondings and chemical change. Total 527 subjects were probed through Interview about Instances (IAI) or Interview About Event (IAE) research instruments in both phases (I & III). Total 26 instances/non-instances /events about all the four selected concepts were used to explore students understandings of each subjects. The reliability of the instrument (IAI & IAE) was determined by Cohan Kappa through inter-rater reliability. Content validity was established by experts. Overall high frequencies of alternative conceptions or lower frequencies of scientific responses were obtained from the boys and girls subjects of class 10 and class 9 (control groups). These both groups were taught through traditional textbook approach. Although, there is relatively low frequency of alternative conceptions in girls than boys. But overall high proportion of alternative conceptions in boys/girls at secondary level is big challenge for science educators. However, low frequency of the alternative conceptions in experimental groups of both boys and girls determined the effectiveness of constructivist approach towards promoting conceptual change.