To investigate the differentiation mechanism of grass carp preadipocytes, a primary adipocytes culture system was established. Confluent preadipocytes were induced to differentiation, and the morphology and gene expression were evaluated at different stages. It was shown that preadipocytes were gradually filled with droplets and the cellular lipid content increased during the differentiation. Ultrastructure observation indicated that the number of mitochondria increased with adipocytes differentiation.Consistently, the mitochondrial protein content was elevated in the differentiating adipocytes. q RT-PCR showed that the expression level of lipogenesis-related genes such as peroxisome proliferator activator receptor c(PPAR c),lipoprotein lipase(LPL), fatty acid synthase(FAS) and stearoyl-Co A desaturase(SCD) increased during adipocytes differentiation. The mitochondrial relevant gene also elevated when adipocyte differentiation, such as PPAR coactivator-1(PGC-1 a), PGC-1 b and nuclear respiratory factor(NRF-1). However, the expression of carnitine palmitoyltransferase I a(CPT-1 a) gene decreased at the initial stage, but increased at the last stage of cell differentiation. These results indicated that the differentiation process of grass carp preadipocytes is similar to that of landanimals, but the molecular mechanisms are not exactly the same. The findings revealed in this study provides new information to the study of fish adipocyte differentiation. To investigate the differentiation mechanism of grass carp preadipocytes, a primary adipocytes culture system was established. Confluent preadipocytes were induced to differentiation, and the morphology and gene expression were were evaluated at different stages. It was shown that preadipocytes were gradually filled with droplets and the cellular Ultrastructure observation indicated that the number of mitochondria increased with adipocyte differentiation. Consistently, the mitochondrial protein content was elevated in the differentiating adipocytes. q RT-PCR showed that the expression level of lipogenesis-related genes such as peroxisome proliferator activator receptor c (PPAR c), lipoprotein lipase (LPL), fatty acid synthase (FAS) and stearoyl-Co A desaturase (SCD) increased during adipocyte differentiation. The mitochondrial relevant gene also elevated when adipocyte differentiation, such as PPAR coactivator- 1 (PGC-1 a), PGC-1 b and nuclear r However, the expression of carnitine palmitoyltransferase I a (CPT-1 a) gene decreased at the initial stage, but increased at the last stage of cell differentiation. These results indicated that the differentiation process of grass carp The findings revealed in this study provides new information to the study of fish adipocyte differentiation.