This study investigated the decomposition characteristics of acetaldehyde using an electron beam. The removal efficiency (RE) in air, O-2, N-2 and He atmospheres at 10 kGy were 88, 89, 94 and 35 %, respectively. By varying the initial concentration (C-0), G-values at 240 ppm (C-0) were maintained from 6.4 to 7.0 molecules/100 eV, while the G-values at 34 and 60 ppm (C-0) decreased from 4.5 to 1.1 and from 6.6 to 2.0 molecules/100 eV when the absorbed dose increased from 2.5 to 10 kGy. The RE of acetaldehyde at 96 % relative humidity was approximately 10-15 % higher than that at dry air when the absorbed doses were 5-10 kGy. Increasing the water supply did not provide additional improvement of the RE at 2.5 kGy. CO, CO2, O-3 and trace VOC compounds such as C2H4O2, C7H6O, C6H6, C7H8 and C8H10 were detected as by-products.