The radiolytic decomposition ancl mineralization of alachlor with reclucing 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 aqlleous sample saturated with N2O showecl the highest degradation rate and enhanced TOC removal efficiency. The results showed that ·OH radicals inclllced by the reaction of N2O with e(aq)-and/or ·H play the important role for alachlor removal.