Background and objectives: The purpose of this study was to investigate the effect of hip joint abduction an-gle change on trunk muscle activities during plank exercise. Materials and methods: This study was conducted with 34 subjects but 4 subjects dropped out during the measurement and 30 subjects completed the study. Muscle activities were measured while performing plank exercise with hip joint abduction angles at 0, 15, 30, or 45 degrees. Electrodes were placed on four muscles: rectus abdominis, external oblique, internal oblique, and erector spinae muscles. Two-way repeated measures ANOVA was used to analyze the interaction between hip joint abduction angle and trunk muscle activities. One-way repeated measures ANOVA was performed to determine trunk muscle activities of each muscle at each hip joint abduction angle. Eta-squared (\eta2) values were used to determine effect sizes. Results: A significant interaction was found between hip joint abduction angle and muscle type (p 0.05). Hip joint abduction angle was significantly associated with rectus abdominis and erector spinae activities (p 0.05). Conclusions: When the hip joint is abducted during plank exercise in healthy young adults, the muscle activities of rectus abdominis and erector spinae are inversely proportional to hip joint abduction angles.