A wafer processing apparatus includes a reaction tube extending in a vertical direction, a door plate positioned under the reaction tube to seal the reaction tube. The door plate may be configured to load a boat into the reaction tube and support a plurality of wafers. The wafer processing apparatus may include a cap plate on the door plate, the cap plate including a cylindrical body. The cylindrical body may surround a lower side surface of the boat. A guiding recess may be formed in an outer surface of the cylindrical body along a circumferential direction of the cylindrical body. The wafer processing apparatus may include an exhaust portion configured to remove the first gas from the reaction tube through the guiding recess.