This research sought to investigate the effect of the effect of task-based language teaching on motivation and grammatical achievement of EFL junior high school students of Ahvaz. To fulfill the objectives of the study a Homogeneity test (Oxford Quick Placement Test) was administered among 100 students at the junior high school and finally 80 participants were selected. Then, they were divided into two subgroups, namely control and experimental groups. Before starting the treatment, a validated teacher-made grammar test in terms of the materials supposed to be covered in both groups was administered to them as the pre-test. Moreover, a motivation questionnaire were given to both groups at the beginning and at the end of study. Then the experimental group received the treatment, which was teaching and learning grammar through using task-based language teaching and the control group received traditional teaching which is teaching grammar through instruction on examples and drills proposed by the teacher. After 12 sessions of treatment, the two groups were administered the same teacher-made grammar test as posttest. Data were analyzed by Paired and Independent Samples t-test. The findings showed that the experimental group significantly performed better than the control group. Generally, the experimental groups outperformed the control groups. Furthermore, the results of motivation questionnaire show that there was a significant difference between the experimental and control groups' motivation in the post-test of questionnaire which implies that the experimental group's motivation increased significantly. The results suggest that task-based language teaching can be used in English classes to develop grammar ability among Iranian EFL learners.