Abstract: The photochemical transformation of diazepam, one of the most known benzodiazepines, has been conducted in aqueous solution to investigate the removal rate, identify the reaction products and evaluate their toxicity. Diazepam can be effectively removed by UV light photolysis at 254 nm, with the quantum yield of 2.32(0.17)10-3 mol/Einstein. Diazepam has been found to be relatively stable in 90 min simulated sunlight irradiation (300-400 nm). Sixty-five products were identified for UV photolysis of diazepam using a high-resolution accurate-mass TripleTOF system. Among them, 49 products were identified in positive electrospray ionization mode, and 16 in the negative electrospray ionization mode. On the basis of the products identification and phototranformation behaviour, 8 plausible reaction pathways were proposed for degradation of diazepam during UV photolysis process. Structural changes to the diazepam molecule included hydroxylation, hydrolyzation, N-demethylation, loss of phenyl group, benzodiazepine ring rearrangement and contraction. Toxicity evaluation using USEPA EPI Suite? software indicated that the toxicity of the main phototransformation products was increased than that of the parent compound diazepam.