The biopretreatment of municipal solid waste by a combined hydrolytic-aerobic process has become of great interest for biostabilization or biodrying.The study aimed at investigating the effect of leachate inoculum on the biopretreatment.During the hydrolytic stage, the inoculum addition at the ratio of leachate to waste (LWR) of 5% or 7% stimulated the start-up of hydrolysis and enhanced the hydrolysis rate by 27.4% or 24.2%; whereas the inoculation at LWR of 1% had almost no effect on the hydrolysis rate and the inoculation at LWR of 10% reduced the hydrolysis rate by 12.7%.During the subsequent aerobic stage, the inoculations at LWRs greater than 5% decreased organics degradation rate.As a whole, compared with none inoculation, the total degradation rates of organics for inoculating trials at LWRs of 5%, 7% and 10% decreased by 14.5%, 14.3% and 32.7%, respectively.Correspondingly, their net water removal rates were reduced by 4.4%, 5.8% and 19.0%.The inoculation at LWR of 1% could not significantly affect the biopretreatment.The inoculum addition at LWR of 5% or 7% could shorten hydrolytic stage and thus accelerate the whole combined process.Moreover, the inoculations at LWRs greater than 5% were favorable for lignocelluloses degradation.