The present invention provides a dishwasher and a control method thereof, by which water of high pressure can be effectively supplied to spray nozzles with a small-sized motor having a small maximum output. The present invention includes a tub receiving tableware therein, a plurality of spray arms situated at different heights arms within the tub, respectively, the spray arms including upper and lower arms spraying water on the tableware, and a sump assembly including a sump housing storing the water therein, a pump coupled with the sump housing to pump the water, and a diverting valve alternately guiding the pumped water to the upper and lower arms. The present invention includes the steps of operating a pump motor at a predetermined RPM, alternately supplying pumped water to spray arms arranged at different heights within a tub, respectively, deciding whether the water is supplied to the spray arm arranged at a higher position, and operating the motor at an RPM higher than the predetermined RPM if the water is supplied to the spray arm arranged at the higher position.