The radiolytic decomposition and mineralization of alachlor with reducing radical scavenger gases such N2O and O2 were studied. Alachlor (10 mg/L) was completely degraded with the irradiation dose of 500 Gy. The TOC removal efficiencies of alachlor in aqueous solutions saturated with no gas (control), N2O and O2 were 48, 87 and 70％, respectively, Alachlor aqueous sample saturated with N2O showed the highest degradation rate and enhanced TOC removal efficiency. The results showed that. OH radicals induced by the reaction of N2O with e(aq)- and/or ·H play the important role for alachlor removal.