| Summary: | A 12-week rearing trial was carried out to estimate effects on the growth performance, physicochemical indexes, quality, and the molecular expression of Yellow River Carp (<i>Cyprinus carpio haematopterus</i>) using five practical diets, including dietary protein levels of 220, 250, 280, 310, and 340 g/kg. The results illustrated that the fish’s weight gain (WG) and specific growth rate (SGR) were significantly influenced, with an ascending dietary protein level of up to 250 g/kg (<i>p</i> < 0.05). The carp muscle contents of total saturated fatty acids (∑SFA), monounsaturated fatty acids (∑MUFA), polyunsaturated fatty acids (∑PUFA), and fatty acids (∑FA) decreased significantly with the ascending dietary protein levels, except for the 250 g/kg protein diet (<i>p</i> < 0.05). Only the glutamic acid and total essential amino acid (∑EAA) contents were significantly influenced by the ascending dietary protein levels (<i>p</i> < 0.05). The relative <i>GH</i> expression of the carp muscle significantly decreased with the increase in the dietary protein level up to 310 g/kg, and then it significantly increased (<i>p</i> < 0.05). In the intestines, the peak relative <i>TOR</i> expression was observed on the 220 g/kg protein diet, while the relative <i>4EBP1</i> expression was significantly influenced by the dietary protein level up to 250 g/kg (<i>p</i> < 0.05). In the muscle, the peak relative <i>TOR</i> and <i>4EBP1</i> expression levels were observed on the 250 g/kg protein diet. In gills, the lowest relative <i>Rhag</i>, <i>Rhbg</i>, and <i>Rhcg1</i> expression levels were observed on the 250 g/kg protein diet. Based on all of the aforementioned results, the optimal dietary protein level for <i>Cyprinus carpio haematopterus</i> (160.24 ± 15.56 g) is 250–280 g/kg.
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