Abstract: In this study, we examined the tolerance to carbonate alkalinity of two sizes of Nile tilapia (Oreochromis niloticus) Large: (21.56±0.27) g, Small: (5.92±0.03) g. Different carbonate-alkalinity solutions were prepared with NaHCO₃ to carry out the experiment: 1) The fish were directly transferred from fresh water to 30 or 22 g·L⁻¹ carbonate alkalinity solutions to determine the mortality. 2) The fish were acutely exposed to different carbonate alkalinity solutions ranging from 6 to 10 g·L⁻¹ for 96 h. 3) For chronic alkaline acclimation experiments, the carbonate concentrations increased gradually at three levels of +2, +4 and +6 g·(L·d)⁻¹. The results show that the half lethal alkalinities of Nile tilapia under acute stress for 96 h ranged from 6.25 to 9.01 g·L⁻¹. The alkaline tolerance of Nile tilapia was weaker than that of fish which adapt to extreme alkaline environment (e.g. Gymnocyprinus przewalskii), but it was still stronger than that of most freshwater cultured fish. At juvenile stage, the tolerance of Nile tilapia to carbonate stress was significantly enhanced when its body mass increased more than three times. Nile tilapia can be acclimated by daily alkaline increment of 2 g·L⁻¹. The relationship of "time-alkalinity-mortality" under acute alkaline stress can be well fitted by complementary log-log model.