The government of Kenya implemented Free Primary Education (FPE) in January 2003. The main objective of this programme was to provide education to all primary school children irrespective of their economic backgrounds. Academic performance in Public Primary Schools in Nakuru Municipality has been declining over time since the introduction of (FPE). This study assessed the influence of class size on classroom interaction during Mathematics discourse in the wake of FPE in Public Primary Schools in Nakuru Municipality. The study adopted ex post facto research design. It targeted all the 59 Public Primary Schools in Nakuru Municipality. Simple random sampling was used to select 4 schools for the study. Mathematics lessons in Classes 1 and 6 were chosen from the selected schools to represent the lower and upper classes respectively. The data were processed and analyzed using descriptive and inferential statistics with the aid of Statistical Package for Social Sciences (SPSS). The findings of the study revealed that FPE policy has led to increased enrolments in schools which have created increased class sizes and Pupil-Teacher Ratios. The increased class sizes have influenced teacher-pupil interaction and pupil-pupil interaction.