Abstract : Cooperative learning is an educational approach which is a highly recommended instructional procedure. This study examined the effect of cooperative learning on students’ achievement in the unit of Fractions in Mathematics for Fourth Grade. For this purpose, two samples were taken (each consisted of 46 students) the first one from Jaba Girls Basic School in Palestine, and the other one from Mobes Girl School in Jordan. Each one of them was divided into two groups: an experimental group and a control group, cooperative learning was applied to the experimental group and the control group was taught by teacher-centered teaching. The students’ achievement was measured by a researcher-designed achievement test given to students as a pretest and a posttest. Analysis of the achievement test scores responses revealed that cooperative learning strategy have significantly more positive effects on students’ achievement than teacher-centered teaching method. These results provide an evidential base for educational authorities to implement cooperative learning methods and encourage and persuade teachers to implement these methods in the two countries mentioned before