A laundry treating apparatus may include a main drum and an auxiliary drum selectively mounted on or separated from the main drum. A method for controlling the laundry apparatus may include rotating the main drum to cause a centrifugal force for discharging water from the auxiliary drum before supplying washing water into the auxiliary drum. The method may include one of washing laundry in the main drum while the auxiliary drum is separated from the main drum, washing laundry in only the auxiliary drum while the auxiliary drum is mounted on the main drum, or washing laundry simultaneously in the auxiliary drum and the main drum while the auxiliary drum is mounted on the main drum.