PURPOSE: The purpose of this study was to determine the effects of the length of the pectoralis minor on muscle activity of trapezius and pectoralis major in subjects in subjects with shortened pectoralis minor muscle. METHOD: The subjects was participated in 36 with shortened pectoralis minor muscle. All subjects was examined the length test of pectoralis minor muscle. we divided by 3 groups. group Ⅰ(n=12) was for 4∼5cm of length of pectoralis minor muscle, group Ⅱ(n=12) was for 5∼6cm, group Ⅲ(n=12) was for above 6cm. The EMG activity of upper trapezius, middle trapezius, lower trapezius and pectoralis major muscle activity was measured by surface EMG while elevationg the right arm in sitting postion with head to the neutral, shoulder elevation 135° with scaption. Data were analyed using one-way ANOVA with a Tukey post hoc test. RESULT: The EMG activity differed significantly among the three groups(p<.05). The group Ⅲ had significantly greater EMG activity of upper trapezius and pectoralis major muscles than group Ⅰ and Ⅱ(p<.05). Also, The group Ⅲ had significantly smaller EMG activity of lower trapezius muscle than group Ⅰ and Ⅱ(p<.05). But, these was no significant difference in the EMG activity of the middle trapezius muscle among the groups (p.05). CONCLUSION: Therefore, the result of this study should be suggested that the shortened pectoralis minor muscle was affected the EMG activity of the upper trapezius, lower trapezius and pectoralis major. Ultimately the length of the pectoralis minor muscle leads to the muscle imbalance in shoulder girdle.