The study proposed an optimal cooperative learning grouping technique to facilitate medical university students' English learning process, considering their complementary competencies. A quasi-experimental design was adopted to evaluate the feasibility of using the grouping technique. TOEIC (Test of English for International Communication) pretest and posttest were given to both the experimental group (32 students) and the control group (31 students) before and after a 16-week intervention. After the intervention, the posttest results showed that the experimental group had significantly higher mean scores than the control group. As for effect sizes, there were moderate effects on listening, reading, and writing competencies and a large effect on speaking competency. The results confirm that the proposed optimal cooperative learning grouping technique is effective for improving students' English learning. In other words, the grouping method considering students' complementary competencies is worth recommending for cooperative learning.