Propiconazole (PRO) is a triazole fungicide that is frequently detected in the water. In this study, we investigated the kinetics and degradation mechanism of PRO during the UV photolysis and UV/H2O2 processes. PRO was removed by the pseudo-first-order kinetics in both processes. The removal of PRO was enhanced by increasing H2O2 concentration in the UV/H2O2 process. The highest removal under neutral conditions, and lower removal of PRO were observed in acidic and alkaline pHs in the UV/H2O2 process. The presence of natural water ingredients such as Cl−, NO3−, humic acid acted as radical scavengers, but HCO3− ion acted as both radical promoter and scavenger in the UV/H2O2 process. The transformation products (TPs) of PRO during both processes were identified using LC-QTOF/MS. Four TPs (M+H+ = 238, 256, 306, and 324) were identified during UV photolysis, and six TPs (M+H+ = 238, 256, 306, 324, 356, and 358) were identified in the UV/H2O2 process. Among the identified TPs, TP with M+H+ values of 356 and 358 were newly identified in the UV/H2O2 process. In addition, ionic byproducts, such as Cl−, NO3−, formate (HCOO−), and acetate (CH3COO−), were newly identified, indicating that significant mineralization was achieved in the UV/H2O2 process. Based on the identified TPs and ionic byproducts, the degradation mechanisms of PRO during two processes were proposed. The major reactions in both processes were ring cleavage and cyclization, and hydroxylation by OH radicals. The Microtox test with Vibrio fischeri showed that, while the toxicity of the reaction solution increased first, then gradually decreased during UV photolysis, the UV/H2O2 process initially increased toxicity at 10 min due to the production of TPs, but toxicity was completely removed as the reaction progressed. The results obtained in this study imply that the UV/H2O2 process is an effective treatment for eliminating PRO, its TPs, and the resulting toxicity in water. Display omitted •Degradation of propiconazole (PRO) in UV photolysis and UV/H2O2 were examined.•The water components mostly acted as radical scavengers in the UV/H2O2 reaction.•Six transformation products (TPs) of PRO during both processes were identified.•Ionic byproducts as mineralization were identified in the UV/H2O2 process.•The degradation mechanisms of PRO during two processes were proposed.