Introduction: This article examined the effect of multimedia instruction on students' learning outcomes (achievement and interest) in secondary school physics. Method: The sample comprised of 517 (294 boys and 223 girls) students who came from 12 senior secondary schools in Ibadan Educational Zone 1, Oyo State, Nigeria. Their ages ranged between 15 and 17 years. There were three experimental groups and a control group. Three courseware versions were developed to examine the interface effects. A conventional lecture method group served as control. The moderating effect of mathematics achievement level on students' learning outcomes in physics was also examined. Data were analyzed using Multivariate Analysis of Covariance (MANCOVA). Results: Results showed that students in the animation + narration + on-screen text group had highest post mean scores in physics achievement test and interest in physics inventory. Students who learnt physics in computer-based multimedia environment had better learning outcomes in physics than the colleagues who learnt physics under teacher-based environment. Also, students with high math achievement level performed better in physics achievement test than their colleagues in low math achievement level group. Interaction effect of treatments and math achievement level was not statistically significant. Discussion and Conclusions: Results point out that concurrent use of animation, narration and on-screen text in an instructional interface may help maintain students' interest in multimedia learning environments. More importantly, results show that integrating animations, narratives, and textual information in computer based environment may help to improve students' learning outcomes in physics. The results failed to validate Richard Mayer's redundancy principle of multimedia learning. (Contains 3 tables and 1 figure.)