Abstract: Leydig cells from 2-3 week old piglets were used to study the mechanism of hCG regulating Steroidogenic acute regulatory protein(StAR). The results of this experiment were as shows : (1)Both of hCG and 8-Br-cAMP could enhance both the expression level of StAR protein and that of its mRNA ,increase active ERK1/2 clearly depending on stimulating time. However, the effect of 8-Br-cAMP is faster than that of hCG. (2) While appending the inhibitior of PKA ( PKI) to leydig cells in culture , both of the expression level of StAR protein and that of its mRNA began to drop significantly and the activity of ERK1/2 was inhibited but the level of StAR mRNA could still be detectable.(3) While appending the inhibitior of MAPK (PD98059), the expression level of StAR protein mRNA dropped significantly .These results inferred that at the beginning of hCG stimulating, hCG increased the level of StAR protein by cAMPPKA. With the increase of time, hCG increased the level of StAR protein through cAMPPKA ERK1/2