This study aimed at investigating the effect of cooperative learning on developing the sixth graders' decision making skill and their academic achievement. The study sample, which was selected randomly, consisted of (46) students and divided into two groups: the experimental group that taught using the cooperative learning strategy and the control group that followed the traditional method. The students were taught the Geometry unit in mathematics from their mathematics book which required (16) lessons to finish. Then, an achievement and a decision making skill tests were administered. The validity and reliability of the tests were achieved by using suitable methods. The results of the study showed that there were significant statistical differences between the two groups concerning the students' skill in decision making and their results in the achievement test in favor of the experimental group.