Haloacetonitriles (HANs), a specie of new emerging halogenated nitrogenous disinfection by-products (N-DBPs) including trichloroacetonitrile (TCAN), dichloroacetonitrile (DCAN), monochloroacetonitrile (MCAN) and dibromoacetonitrile (DBAN), were more cytotoxic and genotoxic than the conventional DBPs. In this study, kinetics and mechanisms in the oxidization of HANs by UV/peroxymonosulfate (PMS) process were investigated. The degradation of chloroacetonitriles (CANs) in UV/PMS process was more remarkable than UV alone, PMS alone and UV/H2O2 process. And the order of the rates of degradation of HANs was followed: TCAN > DCAN > MCAN, while DBAN was remarkably degraded by UV alone. The degradation of DCAN was accelerated by humic acid and Cl-, while suppressed by bicarbonate. Scavenging experiments showed that hydroxyl radical ((OH)-O-rad) was the predominant species in the reaction. According to the gas chromatography/mass spectrometer (GC/MS) analysis, the breakage of C-Cl bond was the dominant pathways in degradation of CANs.