Objective: The purpose of this study was to investigate effects of hand grip strength on the muscle activation of shoulder joint in breast cancer patients. Design: Cross-sectional study. Methods: Ten breast cancer patients who agreed to active participation were included. These patients were operated with either conservative surgery or segmental mastectomy, and then were treated with radiation therapy and chemotherapy. The activity of the upper trapezius, lower trapezius, supraspinatus and serratus anterior muscle were measured using surface electrodes during 4 hand gripping tasks (lowered their arms in standing position, 0%, 30%, and 50% of maximum voluntary contraction) in the scapular abduction plane. Results: The results were analyzed using a one-way repeated measures ANOVA. There was a significant difference in the lower trapezius and supraspinatus muscles according to grip strength, lower trapezius and supraspinatus muscles showed significantly difference according to grip strength (p<0.05). The result of the muscle activation according to hand strength (0%, 30%, and 50%) it showed a significant difference between the upper trapezius and supraspinatus in 0% grip strength (p<0.05). In addition, it did not show a significant difference between muscles in 30%, 50% hand strength. Conclusions: This study showed an increase in shoulder muscle activation with increasing hand grip strength with the upper trapezius muscle being more activated than other muscles in 0% grip strength. The finding of this study suggests usefulness for development of preventative measures and rehabilitation strategies for increasing shoulder motor function in patients with breast cancer.