To identify the effect of cooperative learning (CL) on the achievement's of EFL learners, a 12-week study was conducted in four secondary schools in Saudi Arabia. This study aims to examine the effectiveness of cooperative learning (CL) in developing English as a Foreign Language (EFL) students' grammatical competence in a relatively under-researched context (i.e., Arabia). In particular, this this study aims to investigate the impact of cooperative learning in comparison to traditional instruction in learning English grammar on the achievement of students. This study contributed to the knowledge about how students learn English grammar as a foreign language when they work cooperatively together in cooperative learning groups in comparison to peers who work in traditional classrooms (small groups). The participants in this study were 139 tenth grade male students, aged 14-15 years, in four boys' secondary schools in Al-Baha city.The results showed that there are statistically significant differences between the mean scores of the students who were taught English in the cooperative learning environment (the experimental group), and those who were taught the English by using the traditional small group method (the control group) in the post-test. This difference was in favour of the experimental group. However, the results showed that are not statistically significant differences between the students in the experimental classes and the students in the control groups in their English achievement test score at the pre-test.