Abstract: To investigate the effects of water pH on the growth, nutrient composition and fillet quality of Nile tilapia (Oreochromis niloticus), three groups (low pH, control and high pH) were set with pH levels 4.0, 7.2 and 8.5, respectively, for 10 weeks trial, and then sample were collected for assessing the growth, serum biochemical parameters, muscle nutritional composition and fillet quality related parameters. The results showed that the high and low water pH caused a significant decrease in the weight gain and the content of serum triglycerides of Nile tilapia (P<0.05), and a significant increase in liver glycogen accumulation and the activities of serum aspartate aminotransferase and alanine aminotransferase (P<0.05). In addition, high water pH resulted in a significant decrease in viscerosomatic index and muscle adhesiveness of Nile tilapia (P<0.05), but a significant increase in the muscle cooking loss, muscle lactic acid content and serum lactic acid content (P<0.05). Moreover, the relative content of muscle PUFA, especially n-3 PUFA, increased significantly in the high pH group (P<0.05), indicating the nutritional value of muscle was improved. The low water pH resulted in a significant increase in the viscerosomatic index and feed conversion rate of Nile tilapia (P<0.05), and a significant decrease in muscle springiness (P<0.05). In conclusion, in addition to increasing the muscle n-3 PUFA content, neither high nor low water pH is suitable for the healthy growth and high flesh quality of Nile tilapia.