This study was conducted to develop and validate a computer-assisted instructional (CAI) package on physics for senior secondary school students in Nigeria. Teaching of science subjects in Nigeria is mostly done using traditional method and which was identified as a major cause of students’ poor performance in physics. Several researches have indicated (i) Equilibrium of Forces (ii) Simple Harmonic Motion to be some of the topics responsible for students’ poor performance in Senior Secondary School Certificate Examinations (SSSCE) in Nigeria. Studies have proven the efficacy of CAI for improving students’ performance in all disciplines, developing one for Nigerian physics students is inevitable. The package was developed in an html format using Macromedia Dreamweaver as the overall... platform. Other computer programs utilized during the development process are: Microsoft Word, Macromedia Fireworks 8, and Macromedia Flash 8. The package was developed in accordance ADDIE model. The validation was done in four stages: content validation (physics teachers); expert’s validation (computer programmers and educational technology specialists); individualized validation (one-on-one validation by students) and cooperative validation (group validation by students). The observations, comments and suggestions during the validation were used to modify the package. At the completion of the package, development and validation was found to produce a very good performance when used for physics instruction.