The wall erosion characteristics of a circulating fluidized bed reactor(0.25 m-L×0.62 m-W×10 m-H) was observed with various operating conditions, such as fluidizing velocity, total solid inventory and primary to secondary air ratio. The erosion rate was affected by solid hold-up and solid circulation rate as well as fluidizing velocity which were varied due to operating conditions. The wall erosion in dilute phase of the CFB riser increased with increasing solid hold up resulted from increasing of the fluidizing velocity and the primary to secondary air ratio. The erosion of the wall in dense phase increased with increasing fluidizing velocity although the solid hold up decreased with fluidizing velocity. The extent of erosion in the transition phase was somewhere in between the dense and the dilute phases. The erosion of the roof of the CFB riser increased with increasing solid circulation rate resulted from increasing the fluidizing velocity, total solid inventory and primary to secondary air ratio. The middle part of the roof where the solid circulation rate was higher than that of the side part of the roof, showed the highest erosion rate. The erosion seemed to be serious at the area around the cyclone inlet duct due to the enlarged down flow of the solid particles at this area.