This study investigated the biodegradability of fresh leachate from a young municipal solid waste landfill at the inoculum to substrate ratios (V/V) of 10/100, 25/100, 50/100 and 100/100, as well as the transformation of organic matters in leachate under prolonged hydraulic retention time (HRT) of 50 d. Fresh leachate showed a good biodegradability, with chemical oxygen demand (COD) removal efficiency as high as 87%-92% and cumulative methane yield close to theoretical value (0.35 L CH4/g CODdeg). Methane production and COD depletion presented biphasic characteristics associated with the successive utilization of two major intermediates, acetate and propionate. The biphasic degradation of fresh leachate was resulted from the different hydrolysis rates of diverse substrates and the changes in microorganism community structure. After 50 d, the effluents were dominated by high-MW organic compounds (MW > 10 kDa) at each inoculum ratio, which might be refractory compounds released from cell lyses.