The fish feed industry continues to researches for optimum diet demands for candidate species culture. For this purpose, in vitro analyze methods may be more efficient than in vivo assays. This study includes two different stages; the first one is about the effect of commercial feeding protocol on protease enzyme alteration and the second one is about the inhibitory effects of different protein sources on early life proteases of Russian sturgeon, A. gueldenstaedtii, juveniles. In the first step, feeding with live prey and transition period to artificial feed significantly affected the daily amount of protease in digestive system ( P < 0.05). In the second part, some protein sources used in micro diets were tested in vitro for examination of their possible inhibitory effects on the proteases of Russian sturgeon larvae. The minimum inhibitory effect was obtained from fish meal (15.44%), but however, soybean protein concentration, soybean meal, corn gluten and rice bran inhibited the proteases significantly higher than FM and its combinations (63.55, 71.81, 72.24, and 80.77%, respectively). Additionally, dual combinations between fish meal and soybean meal/soybean protein concentration with the ration of three to one ( 3: 1) was moderate (26.38 and 22.13 %), whereas blood meal extremely produced a 97.28% inhibitory ratio.