This research investigated the effect of class size on academic achievement of chemistry students secondary schools in public in Ikere local government area of Ekiti State. The study was guided by four (4) objectives which were raised into three (3) research questions and formulated into four (4) hypotheses. The study adopted quasi – experimental research design. The sample size of eighty (80)respondents were used for the study using purposive sampling technique to select a school that practices co-educational school with at least 100 Chemistry students in the SS2 classes. The class was grouped into two. Group 1 named large class consisted of 50 students while the second group, Group 2 was named small class with 30 Chemistry students. The instrument for data collection in the study was a student Chemistry Achievement Test (SCAT). The test retest method of reliability was used to ascertain the reliability index of the instrument using Pearson’s Product Moment Correlation (PPMC). A coefficient of 0.85 was obtained. Frequency counts and percentage were used to analyse the demographic data of respondents while the hypotheses were tested using inferential statistics of t-test. Based on the findings of the study, the researcher concluded that Chemistry class size either small or large class size had no effects on students’ academic achievement but students’ achievement in either small or large class size could be affected by factors like teachers’ effectiveness, teaching method, instructional materials available for use etc. Students’ treatment in either large or small class size had no effects on Chemistry achievement scores of students taught in small class and those in large class in the post-test. Class size did not affect Chemistry achievement scores of male and female students taught in small class in the post-test. Gender Chemistry achievement is independent of large class in the post-test. Therefore, based on the findings, appropriate recommendations were made.