Introduction & Objective: Breast augmentation is the most common aesthetic surgical procedure in the world. Choices of incisions, pocket plane, and implant characteristics, including shape, texture, filler, and volume, constitute the key decisions in surgical planning. There are few data on their impact on the pectoralis major force. Since the function of this muscle is important in daily activities of patients, we design this study to compare the effects of these two approaches on the power of pectoralis major muscle. Materials & Methods: Twenty female patients were included in this clinical trial. Half of them (10 patients) underwent sub glandular breast augmentation and half the other (10 patients) underwent sub-pectoral breast augmentation. The power of pectoralis major muscle were measured before and six weeks after the surgery. The findings were compared, using SPSS software version 16 and the Analysis of Covariance (ANCOVA) statistical test. Results: The analysis revealed that both groups had experienced a reduction in their muscle forces .Investigations also showed that those patients who underwent sub pectoral breast augmentation, suffered from greater decrease in their pectoralis major muscle power (P = 0.03). Conclusions: Although the effect of synergistic muscles and short period of this study are a bias, but our study reveals that sub-pectoral prosthesis insertion decreases the pectoralis major power, relation to the other cases.