Abstract: This experiment was conducted to study the developmental changes of myofiber types in longissimus dorsi (LD) muscle of Rongchang (RC) and Duroc×Landrace×Yorkshire (DLY) pigs under different dietary nutrient condition, by determining the proportion of mRNA abundance of four isoforms of myosin heavy chain (MyHC I, 2b, 2b and 2x) with semiquantitative RTPCR. The results showed that: (1)From 10 to 20 kg BW, the composition of myofiber types in LD muscle of both breeds changed dramatically with significantly decreased MyHC I and 2x fibers but ramarkably increased MyHC 2b fiber. (2) From 20 to 120 kg BW, the developmental changes of myofibers in LD muscle varied in breed and myofiber type. Except MyHC 2b fiber, the developmental patterns of the other three myofiber types were different between the two breeds. (3)There was no difference in the proportion of four myofiber types in LD muscle during 10-50 kg BW between the two breeds, but at 80 kg BW, RC pigs showed significantly lower proportion of MyHC 2b fiber and higher proportion of MyHC 2a fiber compared with DLY pigs. (4) Dietary nutitional levels had no significant effect on the composition of myofiber types in LD muscle of both breeds. The results suggested that the developmental changes and composition of myofiber types in LD muscle varied in breed. The breed difference of the composition appeared at 80 kg, and RC pigs showed significantly lower proportion of MyHC 2b fiber, which might correspond to their better meat quality.