Myostatin (MSTN) is a negative regulator of skeletal muscle growth and development. The skeletal muscle in MSTN-/- mice is significantly hypertrophied, with muscle fiber type Ⅱ increasing significantly while muscle fiber type I decreasing. However, it is still not clear how the skeletal muscle types change in MSTN-/- pigs, and how the mechanism for MSTN regulates fiber types, especially in large animals like pigs. This study conducted a comprehensive analysis of the composition of skeletal muscle fibers in MSTN-/- pigs produced in our laboratory. It was observed that, compared with wild-type (WT) pigs, both the total mass of skeletal muscle and type Ⅱb muscle fibers increased significantly (P<0.01), while the type I and type Ⅱa muscle fibers decreased significantly (P<0.01), in MSTN-/- Meishan pigs. In addition, to explore the influence of MSTN on muscle fiber type and its regulation mechanism in the embryonic stage, this study selected a few genes (Myf5, Mef2d, MyoD and Six1) associated with muscle fiber type and validated their expression by quantitative RT-PCR. Herein, it was found that Myh7, Myh2, Myh4 and Myh1 can be detected in the skeletal muscle of pigs at 65 days of gestation (dg). Compared with WT pigs, in MSTN-/- Meishan pigs, Myh7 decreased significantly (P<0.01), while Myh4 (P<0.001) and Myh1 (P<0.05) increased significantly. Meanwhile, the increased expression of Myf5 (P<0.05), Mef2d (P<0.01) and Six1 (P<0.05) in MSTN-/- Meishan pigs suggested that MSTN should regulate the directional development of muscle fiber types in the early stage of embryonic development. Thus, at the embryonic stage, the type Ⅱ muscle fibers began to increase in MSTN-/- pigs. These results can provide valuable information not only for pig meat quality improvement, but also for the study of human skeletal muscle development and disease treatment.