Paraffin embedded tissue section and hematoxylin-eosin staining were used to investigate the morphological characteristics of smooth and striated adductor muscles in Crassostrea gigas. Muscle fibers were stained histochemically for NADH-diaphorase and adenosine triphosphatase activity (ATPase) to identify fiber types in smooth and striated muscles. The morphological characteristics of myofibers within smooth and striated muscles were compared by quantifying muscle diameter and cross-sectional area. Muscle fiber diameter in smooth muscle (5.97 μm) was significantly smaller than that in striated muscle (7.41 μm) (P<0.05). Muscle fiber density of smooth muscle (25 485.65±3917.807 n/mm²) was significantly higher than that in striated muscle (16 908.25±3917.807 n/mm²). The cross-sectional area of myofibers in striated muscle (40.45 μm²) was significantly greater than that in smooth muscle (26.25 μm², P<0.05). The result of NADH-TR histochemistry showed that the myofibers in smooth muscle were slow oxidative fibers (type I), as indicated by positive blue reat unstained by NADH-TR. ATPase histochemistry revealed that fibers in striated muscle were intensively stained and the fibers was type Ⅱ with a high ATPase activity. The fibers in smooth muscle were lightly stained or unstained, suggesting the fibers were type I. The results of ATPase/NADH-TR staining were consistent. In this study, we firstly illustrated the morphological characteristics of myofibers in smooth and striated adductor muscles of C. gigas, and compared the differences in contraction and metabolic profiles between smooth and striated muscles. This research not only provides important information for in-depth studies on biological characteristics of muscle fibers in C. gigas, but also provides theoretical basis for improving the meat quality of molluscs.