The study was designed and conducted to determine the effectiveness of Computer Simulations on Senior secondary School students’ achievements in practical physics in Educational district III, Lagos state, Nigeria. A non-randomized pre-test, post-test control group quasi-experimental research design was adopted for the study. A sample of 219 Senior Secondary Two (SSII) physics students, drawn by multistage sampling method from six co-educational schools in Educational district III was used for the study. Three research instruments: Practical Physics Achievement Test (PPAT), Practical Skill Rating Scale (PSRS) and Students’ Attitude Inventory Scale (SAIS) were validated by experts and used to collect data for the study. The data collected was analyzed using Analysis of Covariance (ANCOVA) and Estimated Marginal Means at 0.05 level of significance. Graphical illustrations were used to further explain the interaction effect. The study revealed that the students in the experimental group (Computer Simulations) instructional strategies had a higher mean in both the achievement and acquisition of practical skills than their counterparts did in the control group (Conventional) instructional strategy.