A washing machine for controlling optimal operating revolutions per minute (rpm) of a drain pump in a draining course and a dehydrating course includes a washing tub; a drain pump configured to drain water in the washing tub; a pump motor configured to operate the drain pump; an inverter circuit configured to supply a driving current to the pump motor to operate the drain pump; and a controller configured to control the inverter circuit to supply reference power to the pump motor in response to starting of a draining course, determine reference revolutions per minute (rpm) based on average rpm of the pump motor during a preset period of time, and control the inverter circuit to operate the pump motor at the reference rpm in response to the determining of the reference rpm.