This study examined classroom environment and mathematics achievement of senior secondary school (SS2) students' in the Calabar Cross River State, Nigeria. Ex-post facto quasi-experimental research design was used to test two hypotheses on the influence of two classroom variables, class size and instructional materials on students' academic achievement in mathematics. A sample of 700 students was selected from public secondary schools for the study using stratified and simple random sampling procedures. Two instruments used for data collection were a questionnaire on classroom environment and test items on mathematics achievement. The reliability estimate of the instrument was established through Cronbach Alpha reliability estimate and Kuder-Richardson K-R-20 formula which gave the reliability indices to range from 0.75 to 0.78. Independent t-test was the statistical technique adopted to test the hypotheses at 0.05 level of significance. The result of the analysis revealed that class size and availability of instructional facilities significantly influenced students' academic performance in mathematics among SS 2 students in Calabar Nigeria. Based on this finding, increased government funding for provision of conducive classroom environment, with optimum class sizes is recommended. Instructional materials for teaching mathematics should be made available in public schools.