Abstract: The study found out the relationship between class size and student academic achievement in some selected secondary schools in Southwestern Nigeria. The study adopted the descriptive survey design and simple percentages were used in analysing the data. Two validated instruments (QMT and MAT) were used in collecting data for the study. The subject of the study was made up of 1750 senior secondary school students and 123 mathematics teachers selected from 2 secondary schools in each of the Senatorial districts in Southwestern part of Nigeria. The results showed that the performance of students in large classes was very low (23%) compared to those students in smaller classes (64%). There was difference in the performance of male and female students in either group. It was recommended that policy makers and government should ensure that more classrooms are built and number of students in a class should not be >30.