Modern concept of education is based on students' centered learning approaches where collaborative instructional strategy is an emerging approach. It has been tested in different subjects and its effectiveness has been proved. Therefore, this experimental study investigated the effects of Collaborative Instructional Strategy (CIS) on mathematics achievement of fifth grade students. The experiment was conducted at a Government school in District Swat, Pakistan using pre-test post-test comparative group design on 64 students in two groups (control and experimental). Mathematics Attainments Test (MAT) was developed to measure students' academic achievement. Collaborative mathematics instructional lesson plans (CMIL) were also developed to teach mathematics. The collected data were analyzed though mean, standard deviation, pair sample t test and independent sample t test. The results of the experiment showed that Collaborative Instructional Strategy (CIS) has a significant positive effect on the academic achievement of Primary school students in the subject of mathematics. It was recommended that Collaborative Instructional Strategy (CIS) may be use to teach mathematics at primary level.