In this paper, combustion behaviors of two kinds of raw sewage sludge were investigated under air and pure-oxygen atmospheres, and in the case of nitrogen atmosphere, reaction behavior was also investigated as a reference. The overall combustion process can be described as follows: dehydration and devolatilization simultaneously take place from the surface to the interior, then the produced steam and volatiles flow out from the interior, passing though hot layers and react with the char produced from devolatilization. Consequently, volatiles and syn-gas burn around the pellet surface and increase the surface temperature. When the temperature is high enough to melt the ash layers, some small molten droplets were formed and covered the pellet surface. Finally, the remained char combustion facilitates ash droplet agglomeration. Shrinkage and ash agglomeration are the main contributions for volume reduction of raw sludge. The rate-limiting steps in overall reactions were suggested as mass diffusion of water and volatiles.