To explore the liver fat accumulation and its possible reasons of grass carp (Ctenopharyngodon idellus) fed with broad bean, liver samples of grass carp fed with broad bean (treatment group) and artificial feed (control group) were collected. The pathological feature, fat accumulation and hepatic structure were observed by H&E, oil red O staining and transmission electron microscopy method, respectively. The expression leads of genes associated with fatty acid synthesis were further analyzed. Additionally, some biochemical indexes were also detected both in liver and serum. The results showed that:hepatic steatosis was observed accompanying the abnormal morphology and decreased number of mitochondria in the treatment group; Triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in the serum of treatment group were significantly higher than those of the control group (P<0.05); TG, TC and hydrogen peroxide (H₂O₂) in the liver of treatment group were extremely higher than those of the control group (P<0.01), but catalase (CAT) activity and glutathione (GSH) were extremely lower (P<0.01); The mRNA expression levels of fatty acid synthesis (acc and fas) in treatment group were extremely higher than those in the control group (P<0.01). The results demonstrated that long-term feeding broad bean could cause fat accumulation in the liver of grass carp, which may be related to the increase of fatty acid synthesis, oxidative stress and collapse of mitochondrial structure. This study would provide scientific data for nutritional regulation of crisp grass carp and prevention from fish hepatic steatosis.