The objectives of this study were to investigate the muscle fiber characteristics of the pectoralis major muscle, and its relation to growth performance in the random bred control (RBC) and heavy weight (HW) Japanese quail lines at 42 days of age. The HW line had greater body (232.0 v. 100.2 g, P < 0.001) and pectoralis major muscle (19.0 v. 6.2 g, P < 0.001) weights than the RBC line. Color differences were observed between the superficial and deep regions of the pectoralis major muscle, with the superficial region showing a higher value of lightness than the deep region of the RBC or HW lines (P < 0.001). The percentage of the superficial region in the pectoralis major muscle was higher in the HW line compared with the RBC line (46.2% v. 38.0%, P = 0.017). There were no significant differences in the total fiber number in the superficial and deep regions between the two quail lines (P = 0.718). The HW quail line showed a larger mean fiber cross-sectional area (CSA; 375.5 v. 176.6 μm2, P < 0.001) and type IIA fiber CSA (243.7 v. 131.9 μm2, P < 0.001) than the RBC quail line. The HW line also had greater CSA percentage (60.2% v. 34.2%, P < 0.001) and number percentage (41.6% v. 14.2%, P < 0.001) of type IIB fibers, although there were no significant differences in type IIB fiber CSA between the RBC and HW lines (P = 0.219). Therefore, greater body and muscle weights of the HW line are caused by differences in muscle fiber characteristics, especially the proportion of type IIB fiber and the CSA of type IIA fiber, compared with the RBC line. The results of this study suggest that muscle fiber hypertrophy has more impact on body and muscle weights of the different quail lines than muscle fiber hyperplasia.