During last decades, new methods and approaches were put forward to resolve teaching grammar problem. Among them, Task-Based Language Teaching (TBLT), focusing on learner’s involvement in tasks, tried to prepare efficient learners for successful handling of real-world performance and communication. The present study aims at investigating the impact of Task-Based Instruction (TBI) on grammar learning of elementary EFL Learners and their motivation after implementing TBI. In so doing, 74 Elementary EFL learners were selected randomly and whose proficiency level and grammar homogeneity were established via administering a Nelson grammar proficiency test and Michigan grammar test, respectively. Then, the treatment group enjoyed TBI by implementing different tasks for about fifteen 35-minute sessions. The control group benefited the same amount of grammar instruction, but not through the TBI. At the end, students of both groups were given the Michigan grammar-oriented post-test to gauge the effect of TBLT on improving the grammar proficiency of the learners. The data were analyzed through statistical techniques of t-test. The results revealed that TBI had a significant impact on promoting the grammar proficiency of Iranian elementary EFL learners. Moreover, by analyzing the mean of experimental group, it is understood that the level of motivation was high. The study concluded that TBI instruction on grammar could be used effectively for Iranian language learners especially for the improvement of their motivation.