The study aimed to study the effectiveness of the Cooperative learning method of sixth-grade students' achievements in the decimals unit. A pilot study of forty-five sixth-graders students, arranged into three groups of fifteen students each, for two weeks. One-way analysis of variance shows that there was a statistically significant difference between all group’s achievement in the posttest, and applying (Student-Newman-Kaul's)test indicate that there was a statistical significance at (0.05) of the students' achievement between the traditional method and Cooperative teaching method, no significant difference between Cooperative learning groups.