The present study was aimed to examine the morphological, behavioral changes and histopathological changes in the liver of Nile tilapia, Oreochromis niloticus after acute exposure to methylmercury. The toxicity of methylmercury of Nile tilapia was studied in a static renewal bioassay for 48 hours and 96 hours (96 hours L.C-50 0.22 mg/L). The study was conducted on 40 fish that weredivided into 4 groups A, B, C and D. Group A was considered as control group.Group B, C and D were exposed to sublethal concentrations of methylmercury 0.044 mg/L, 0.055 mg/ L and 0.073 mg/L in tap water respectively. The fish exhibited tail and fin disruption, bulging of eyes, scales erosion, mucous secretion, hemorrhage, color change, slow down swimming, lake of balance, restlessness, swimming side-wise, retardation of opercular movement and quick sudden movements. The fish was anesthetized and liver were removed after 48 and 96 hours for histopathologicalstudies. The histopathological changes observed in liver were degeneration of hemopoietic tissue, destruction of hepatocytes, degeneration of bile duct, inflammatory infiltration, vacuolization, pyknosis, presence of MMCs and necrosis in hepatocytes. It can be concludedthat methylmercury has harmful effects on histology of liver of fish. Therefore, the present work suggests that the exposure to the methylmercury should be prevented to avoid injurious health hazard risks to fish.