The study sought to establish differences in the effect of cooperative learning strategy with and without the instructional manual on the Senior High School students’ performance in Mechanics concepts in Physics in the Berekum Municipality, Ghana. The research design employed in this study was the experimental using the pretest and posttest equivalent control group design. A sample of 93 SHS 2 students, drawn from two intact classes was used. An instrument known as Mechanics Concepts Test (MCT) was employed in data gathering. Mean score, standard deviation; t-test, mean gain and effect size analysis were used to answer the research questions, while the independent sample t-test was used to test a hypothesis. The results revealed that students taught using the cooperative learning strategy with the instructional manual performed significantly better in the Mechanics Concepts Test (MCT) than those taught using the cooperative learning strategy only. Therefore, it is recommended that physics teachers should use the cooperative learning strategy along with the instructional manual in order to enhance the performance of students in secondary schools.