The study examined the effects of two modes of computer-assisted instructional package on solid geometry achievement amongst senior secondary school students in Minna, Niger State, Nigeria. Also, the influence of gender on the performance of students exposed to CAI(AT) and CAI(AN) packages were examined. This study adopted a pretest-posttest experimental design with 3 x 2 factorial design and a sample of 120 Senior Secondary class Two (SSII) students (60 male and 60 female). Computer-Assisted Instructional package of two modes; Animation with Text (AT), and Animation with Narration (AN) were employed as treatment instruments and a Solid Geometry Achievement Test (SGAT) was used as test instrument. A trial test was carried out and a reliability coefficient of 0.78 was obtained using the KR-21. Analysis of Variance (ANOVA) and t-test was used in analysing data collected. The study revealed that, there were significant differences in the post-test mean scores of CAI(AT), CAI(AN) and the control group (F = 11.468, df = 119, p<0.05) and the Scheffe's post-hoc test revealed a significant difference between CAI(AN) and the lecture method groups, favoring CAI(AN), there was no statistically significant difference in the post-test mean scores of male and female students taught using CAI(AT) (t = 0.660, df = 38, p>0.05) and CAI(AN) (t = 1.455, df = 38, p>0.05). Based on these findings, it was therefore recommended that mathematics teachers should be encouraged to use CAI(AN) for meaningful and effective teaching and learning of mathematics.