High-fat diet (HFD) is becoming more prevalent, necessitating the enhancement of fish tolerance and utilization of such diets for efficient use in aquaculture.Our previous research indicated that promoting adipocyte hyperplasia via peroxisome proliferator-activated receptor γ (PPARγ) is considered as a new nutritional strategy to increase HFD tolerance and utilization of grass carp, but feed additives with this effect has not been reported in fish.Caffeic acid phenethyl ester (CAPE), a phenolic compound extracted from honeybee hives, has the function of activating PPARγ.To investigate the effect of CAPE on adipocyte hyperplasia and HFD tolerance and utilization in fish, this study added different levels of CAPE (0, 200, 500, and 800 mg/kg) to HFD to prepare five exptl. diets (control, HFD, HFD + C200, HFD + 500, HFD + C800). 270 grass carp (initial weight: 13.98 ± 0.22 g) were divided into 5 groups and fed for 8 wk.HFD reduced final body weight, weight gain rate, specific growth rate, protein deposition rate and crude protein of grass carp, while the addition of CAPE significantly mitigated these adverse effects.CAPE also significantly reduced the HFD-induced increase in feed conversion ratio.As expected, CAPE promoted adipocyte hyperplasia, leading to healthy adipose tissue remodeling, including decreased immune cell infiltration and promoted the ability of adipose tissue to handle fat as well as reduced lipid accumulation.Addnl., CAPE supplementation alleviated hepatopancreas steatosis by reducing hepatopancreas lipid absorption and synthesis, while also improving the protein sparing of lipid in muscle.Furthermore, CAPE significantly decreased the expression of pro-inflammatory factors in adipose tissue, hepatopancreas and muscle, as well as lowered levels of serum lactic dehydrogenase (LDH), alanine aminotransferase (ALT), aspartate aminotransferase (AST) and low-d. lipoprotein cholesterol (LDL), thereby indicating a pos. impact on inflammatory response.In conclusion, our study systematically evaluated the effects of CAPE on growth, metabolism and inflammation of grass carp fed with HFD, and found that CAPE can promote adipocyte hyperplasia and improve the adverse effects of HFD on grass carp, providing valuable insights for developing effective feed additives to improve the tolerance and utilization of fish to HFD.