Long-term settlement of landfills is one of the important items in the design of protection systems such as caps and leachate collection systems as well as in the technical consideration of feasible redevelopment of a landfill site for building foundations and pavement. Long-term settlement mechanisms cannot be merely dealt with by classical Soil Mechanics concepts, since they are considerably dependent upon the biological decomposition of refuse which distinguishes it from typical soil behavior. Nevertheless, a detailed mechanism by which the decomposition causes settlements has not been found. This paper presents a mathematical model that considers the decomposition process of biodegradable refuse from a geotechnical point of view and suggests the overall compressibility behavior of refuse, including the decomposition effect.