Disclosed herein a hydraulic pressure supply apparatus of an electronic brake system includes a motor coupled to a modulator block having a flow path and a valve for adjusting braking hydraulic pressure therein, the motor having a stator and a rotor disposed to be spaced apart from an inner circumferential surface of the stator; a first gear configured to receive a rotational force from the rotor and rotating together with the rotor; a second gear provided with a piston on one side thereof and concentrically connected with the first gear in a ball screw manner to convert a rotational motion of the first gear into a linear motion; and an anti-rotation unit configured to prevent the second gear from rotating together with the first gear.