The aim of this study was to compare and investigate the effects of increased shoulder external rotation range of motion (ROM) and trapezius muscles activities after three different pectoralis major stretching methods. A total of thirty healthy male subjects participated in this study. They were split into three groups. Each group performed one of the three different stretching methods: 1) Manual stretching by a physical therapist (MS), 2) Active stretching using a ball (BS), and 3) Active stretching be themselves (SS). Trapezius muscle activities were measured by using surface electromyogram (S-EMG). The results were measured using Goniometer equipment for the change of ROM and using EMG equipment for the measurement of muscles activities before and after stretching. The stretches were performed three times, and held for 20 seconds on each occasion. The maximal voluntary isometric contraction (MVIC) was determined for each muscle group in order to represent each stretching as a percentage of MVIC, and allow for standardized comparison among subjects. A one-way ANOVA of variance was used to compare the ΔROM (the change between post- and pre-stretching ROM) and Δ%MVIC (the change between post- and pre-stretching muscle activities) among the three groups. The statistical significance level was set at α=0.05. The results showed that the shoulder external rotation ΔROM values did not produce a significant (p>0.05) difference among the three groups, however the Ball Stretching (BS) group showed the largest increase in ΔROM. All of the groups had lower muscle activity after stretching the upper trapezius and higher muscle activity after stretching the middle and lower trapezius. There was no significant difference among the three groups in muscle EMG activity on the upper and lower trapezius. However, the standardized EMG activity on the middle trapezius was markedly higher (p<0.05) for pectoralis muscle Manual Stretching (MS) than BS or SS. These results suggest that pectoralis muscle Ball Stretching (BS) is useful for shoulder external rotation ROM, while Manual Stretching (MS) is more effective for correcting shoulder posture.