A computer system and a control method thereof, the computer system includes a host unit which includes a plurality of lines, a plurality of memory units to which power is selectively supplied, a power supply which supplies power to the plurality of memory units, a graphic processor which connects with the host unit through the plurality of lines and accesses the plurality of memory unit, and a controller which controls supply of power to the plurality of memory units on the basis of an operation mode of the host unit, controls whether to use the plurality of lines in transmitting data in accordance with the supply of power to the plurality of memory units, and controls the graphic processor not to access the memory units receiving no power among the plurality of memory units.