When applying various evaluation tools that analyze work posture risk through observation, accurate measurement of body flexion angle is very important. This study is to investigate differences of flexion angle at reference positions of 6 participants in the same work posture. Six types of trunk flexion postures(0°, 15°, 20°, 30°, 45°, 60°) were constructed using the Poser 6.0 program. Electromyo- graphy(L3/L4 elector spine left, right) for trunk flexion was determined. As a results, muscle activation revealed no significant differences between male and female, but the most significant differences in trunk flexion angle are seen. In general, the muscle activities of the elector spine increased significantly as the trunk angle increased. In this study, however, the muscle activities was lower in the trunk flexion angle 60°than 30°and 45°. Thus, performing the practical and various study for the trunk flexion angle is re quired for the accurate analysis of working postures.\