A dishwasher and a controlling method thereof are provided. The dishwasher includes a tub, a door, a steam generator, and a dish storage. The tub holds dishes and is opened at one end. The door selectively seals the opened end of the tub. The steam generator is provided outside the tub to generate steam. The steam discharger discharges the steam generated by the steam generator into the tub. The dish storage stores dishes to be sterilized by the steam discharged from the steam discharger.