This study investigated the effects of computer animation on the academic achievement of Nigerian senior secondary school students in biology. The moderating effects of mental ability and gender were also investigated. The pretest-posttest, control group, quasi-experimental design with a 2x2x2 factorial matrix was adopted for the study. One hundred and eighty-nine senior secondary school Year II biology students from two randomly selected Federal Government Colleges in two states in Southwestern Nigeria were the participants. Findings show that there is a significant main effect of treatment on students’ achievement in biology. The computer animation was effective in improving students’ achievement, therefore, computer animation is recommended as a means of teaching biology to students in Nigerian secondary schools.