A landfill leachate, which contains large amounts of mierobially refractory humie substances, was irradiated with ⁶⁰Co γ-rays under several conditions and the modification of its biodegradability was examined. The effects of initial pH and dose rate on the modification were found to be insignificant. The apparent effect was observed for the initial concentration of the humie substances. High biochemical oxygen demand (BOD) value was obtained as the initial concentration was higher. As the initial concentration was lower, in contrast, the decreasing rates of total organic carbon (TOG) and chemical oxygen demand (COD) were high. Therefore two possible designs of the combined radiation-microbial processes with and without dilution are proposed. Both processes require only about 30% of the irradiation dose, compared with a simple radiation process.