A rotor of a turbine cooled by compressed air supplied from a compressor of a gas turbine prevents combustion gas from flowing between platforms and guides compressed air discharged outside the platform toward the blade airfoil. The rotor includes a disk having an outer circumferential surface; a plurality of blade airfoils arranged around the disk; and a plurality of platforms coupled to the outer circumferential surface of the disk, each platform having an outer circumferential surface to receive a corresponding blade airfoil and side-facing surfaces which are spaced apart from each other on adjacent platforms of the plurality of platforms to form a cooling passage through which the compressed air flows outward in a radial direction of the rotor, the cooling passage including a bend in a rotational direction of the rotor. The bend directs the compressed air in a direction opposite to the rotational direction of the rotor.