One of the fundamental tactors in selecting a system of co-combustion of dewatered sewage sludge and municipal refuse is a water content of feed sludge to a furnace, namely dewatered sludge or sludge dried by excess heat from the furnece.Comparison of heat balance between two co-combustion systems was carried out in this study. The feed sludge to System 1 and to System 2 was dewatered sludge and dried sludge, respectively. The two Systems had the same units of a gratefired waterwalled combustion unit, a boiler, a generation unit, an exhaust gas treatment unit and so on except a sludge drying unit for System 2. Following ore the results of this study. A. A range of an autogeneous co-combustion ar ratio is wider in System 2 than in System 1. The co-combustion ratio means a ratio of dewatered sludge incinerated to refuse incinerated by weight. B. Comparison of energy costs between the two Systems reveals that System 2 is better than System 1 when autogeneous combustion is not possible, but there is little difference between the two Systems if autogeneous combustion is realized. C. Even if a low heat value is constant, the higher the water content is, the lower the value of sludge as fuel is. In order to solve the problem, new three definitions of heat value are compared with the low heat value.