This study examines the effectiveness of Lesson Study (LS) in enhancing the writing accuracy of EFL learners. A group of 63 intermediate male EFL learners from Gofteman Language Institute in Ardebil, Iran, divided into two experimental groups and one control group, participated in the research. After taking a grammar pre-test on the target structures, the first experimental group received instruction based on the first cycle of LS, a reflective team-negotiated teaching phase. The second experimental group was given a revised LS-based instruction (second cycle). The control group received traditional instruction without LS. Statistical analysis showed a significant improvement in both experimental groups, with the second group demonstrating even greater gains than the first, indicating that LS-based instruction was more effective across both cycles. The results suggest that LS-driven instruction is valuable not only for its effectiveness and practicality in improving micro-level writing accuracy in the Iranian EFL context but also for supporting ongoing professional development among teachers.