A method of regenerating a selective catalytic reduction catalyst on a diesel particulate filter (SDPF) includes predicting a reducing agent amount oxidized in the SDPF during regeneration of the SDPF if the regeneration of the SDPF is necessary; calculating a quantity of heat generated from the reducing agent amount oxidized in the SDPF; calculating a temperature change from the generated quantity of heat; calculating a target temperature when regenerating the SDPF; and performing the regeneration according to the target temperature.