In this study, the effects of nitrite on the tolerance and physiological response in knifefish (Chitala ornata), an un-obligated air breathing species were investigated at College of Aquaculture and Fisheries, Can Tho university Can Tho Viet Nam. In previous study, the medium lethal concentration (96h LC50) of nitrite to the knifefish was 7.82mM. Fish (25-35g) was exposed at nitrite concentrations of 0mM, 1mM and 2.5mM (equivalent to 0 ,LC₅-96hrs and LC₁₀-96hrs – 5% and 10% of lethal concentration at 96hrs) for 0, 1, 24, 48, 72, 96, 120 and 144 hours with 50 fish/120L of density. In each sampling time, 10 fish in each treatment were taken for blood sampling. Erythrocyte had a significant increase at treatment of 1mM NO2-N, reaching 3.6 millions of cell, while there was a sharp decrease of the erythrocyte number in treatment of 2.5mM, around 2.6 million of cell. Although plasma nitrite increase during exposure, the extracellular nitrite maintained below exposed nitrite levels in water, extracellular nitrate also soared by a number of mM. MetHb increased to 38% at 48h and functional Hb minimalized at 54% of total Hb at that time. Therefore there was no significant effects of nitrite to the increases of metHb and plasma nitrite. However, ion balance was significantly influenced. Plasma chloride decreased from 100 mM to 60 mM while plasma osmolality fell down from 320 mOsm to 230 mOsm, plasma sodium from 130 mM to 94mM, plasma potassium from 8.5 mM to 6.9 mM. The significant changes in ion balance of knifefishare typically different from other fish species. Thus, metHb in fish blood increased at high concentration of nitrite in water which causes bad effects on physiology and health of fish.