Abstract: We studied the effects of salinity stress and diurnal variation on the protease and amylase activity of juvenile grey mullet (Mugil cephalus).Five different salinities were set: S₀ (salinity is 0), S₁₀, S₂₀, S₃₃ (control) and S₄₀, respectively. The results indicate: 1) salinity stress has significant effects on activity of digestive enzymes (P < 0.05).Activity of protease and amylase increased within 0~7 d, then decreased within 7~14 d. At the last day (14^th day), protease activity of S₀~S₃₃ increased while that of S₃₃~S₄₀ decreased. The difference of amylase activity of S₀~S₂₀ is insignificant (P>0.05), and amylase activity of S₂₀~S₃₃ increased while that of S₃₃~S₄₀ decreased significantly (P < 0.05); 2) The diurnal variation in activity of digestive enzyme of M.cephalus reveals that the protease and amylase activity reached the maximum at 12 : 00 and 15 : 00, respectively, while bottomed at 6: 00. Thus, feeding periodically at night can help the fish grow fast and healthily.