Abstract: The settlement mechanism of postclosure landfills and the decomposition rule of organic carbon for MSW are discussed in this paper. The opinion that the settlement induced by biodegradation should be considered separately is put forward. A model of analyzing the biodegradation settlement is presented and is utilized to explain the increase of slope of settlement-logt curves observed in situ. It is concluded that under ideal circumstance, the biodegradation settlement can be expressed as a product of final biodegradation settlement and an attenuation factor. The final biodegradation settlement rate is determined by the content of biodegradable organic carbon and a dimensionless parameter, which is the product of biodegradation rate and filling time. The attenuation factor, is determined by the biodegradation rate and time.