A membrane bioreactor (MBR) with an air-lift bioreactor and gravity flow is applied totreating landfill leachate. More than 99% of BOD5 (biochemical oxygen demand for five days) removal efficiency is achieved with less than 35 mg/L of BOD5 in the effluent at less than 1.71 kg BOD5/m3.d of BOD5 loading rate. When DO (dissolved oxygen) is maintained at the range of 2.3-2.8 mg/L and the loading rate of NH4+-N (ammonium nitrogen) is kept at 0.16-0.24 kg NH4+-N/m3.d, the NH4+-N in the effluent is less than 15 mg/L. However, compared with high removal rates of BOD5 and NH4+-N, the removal efficiency of soluble chemical oxygen demand (SCOD)varies between 70% and 96%. The investigation of molecular weight (MW) distribution has been carried out by the gel permeation chromatography (GPC) so as to understand the fate of organic matters in the MBR treating of landfill leachate. Results indicate that organic matters of the landfill leachate are composed of a high MW fraction (MW of the peak, MWp =11480-13182 Da) and a low MW fraction (MWp =158-275 Da). The high MW fraction is not biodegradable, but can be decreased with microfiltration membrane. The most of the low MW fraction is biodegradable, but the residue of the low MW fraction is able to permeate through the membrane, thus resulting in high SCOD in the effluent of the MBR.