Abstract: This experiment was conducted to investigate the effect of CLA on the composition of myofiber types in skeletal muscle cells in vitro. The primary skeletal muscle satellite cells were used, and when the satellite cells were transformed into muscle cells, different CLA levels (0, 50, 100, 150, 200 μg·mL-1) were added into the cells. After D4, D8 and D12 of culture, the ratios of mRNA abundance of four isoforms of myosin heavy chain (MyHC I, MyHC 2a, MyHC 2b and MyHC 2x) were determined by semiquantitative RTPCR. The results showed that the composition of myofiber types in skeletal muscle cells changed markedly with culture time. From D4 to D12, MyHC 2b type fiber was increased markedly, but the other three types of muscle fibers were decreased significantly. The addition of 50 μg·mL-1 CLA had no effects on the composition of myofiber types. The effects of 100 μg·mL-1 CLA on the composition of myofiber types appeared mainly on D12, while 150～200 μg·mL-1 CLA affected significantly the composition of myofiber types from D4 to D12, with an upregulation of MyHC I and MyHC 2a type fibers, and a downregulation of MyHC 2x and MyHC 2b type fibers. The results suggested that the composition of myofiber types in skeletal muscle cells was influenced by CLA, which depended on its dose and culture time. The effect of CLA on the composition of myofiber mainly was represented by increasing the proportions of MyHC I and MyHC 2a type fibers but decreasing the ratios of MyH 2b and MyHC 2x fibers, which could explain partially why CLA improved pork quality.