The study was undertaken to investigate the "Effects of Class Size and Ability levels on Students? Performance in Mathematics among Junior Secondary School Students in Kaduna Metropolis" to proffer solutions to the problem of poor performance in Mathematics. The study made use of experimental design in which three secondary schools were used with a sample of one hundred and fifty (150) students, ninety (90) students from school I representing large class, forty (40) students from school 2 representing medium class and, twenty (20) students for a small class size. Mathematics Achievement Test (MAT) was used for the students while observation schedule was used for the Mathematics teachers. Three (3) research questions were generated to guide the investigation. Also, three (3) hypotheses were postulated and tested at 0.05 level using Analysis of Variance (ANOVA), T-test and descriptive statistics (Mean and Standard Deviation). The findings revealed that students? performance in Mathematics improved as class size decreased. There was no significant difference in the performance of male and female students irrespective of the class size. The findings also revealed that students in mathematics performed better when taught by teachers who have high qualification and teaching experience. Based on the research findings, it was recommended that stipulated maximum class size of 20-40 in Nigerian schools, be observed, government is to ensure adequate employment of dedicated and qualified Mathematics teachers to teach the subject and tutorial should be encouraged where by students with high ability levels take it upon themselves to instruct their colleagues with low ability levels in Mathematics