Objective of the present study was to diagnose and provide the remedial teaching on the topic of Chemical reaction in science subject. Researcher used purposive sampling technique for the selecting the students. Based on diagnostic test, 100 students were selected who scored 35% or less marks, they were divided into two experimental groups (in each 50 students) by lottery method. Researcher used self-made diagnostic test for data collection. The present research was conducted using experimental research method and 'one group pre-test-post-test' experimental design was implemented in the research. Mean, standard deviation and t-test were used to analyze the data. As a result, Students were making mistakes in writing the names of elements, Students did not know about the valency of elements, Students were writing the electron structure instead of the valency of elements, Students did not understand the process of combination, displacement process, decomposition process and oxidation and reduction process. And based on remedial teaching for the unit ‘Chemical Reaction’, we can say that achievement of experimental group-1 and experimental group-2 was similar before the treatment and the remedial teaching was equally effective in experimental group-1 and experimental group-2.