This study investigated the effectiveness of computer animation and geometry instructional model on mathematics achievement and retention on Junior Secondary School Students in Minna, Nigeria. It also examined the influence of gender on students' achievement and retention. The research was a pre-test post-test experimental and control group design. 40 junior secondary school students were drawn from two secondary schools within Minna metropolis. Stratified random sampling technique was used to select 40 students (20 males and 20 females). The Geometry Achievement Test (GAT) was used for data collection. The reliability coefficient of 0.87 was obtained using Kuder-Richardson (KR-20). GAT was administered to students as pre-test and post-test. The students' pretest and post-test scores were analyzed using t-test statistics. The results indicated that the students taught geometry using computer animation performed significantly better in post-test and retention test than their counterparts taught geometry using instructional model and conventional method respectively. However, there was no significant difference reported in the post-test performance scores of male and female students taught geometry using computer animation and instructional model respectively. These findings indicated that geometry concept in mathematics could be taught and learnt meaningfully through the use of computer animation.