An actuator of a hard disk drive moving a magnetic head to a predetermined position on a disk to record and read out data on and from the disk, including a suspension supporting a slider on which the magnetic head is mounted, an actuator arm having a pivot hole formed in a middle portion of the actuator arm and installed on a base plate of the hard disk drive to be capable of pivoting, in which the suspension is installed at one end portion of the actuator arm, and a voice coil motor having a coil coupled to the other end portion of the actuator arm by interposing a molding portion therebetween and at least one magnet arranged to be separated a predetermined distance from the coil and to face at least one surface of an upper surface and a bottom surface of the coil. In the actuator, a plurality of protrusions protruding toward the molding portion are provided at a contact surface of the actuator arm contacting the molding portion so that a contact area between the molding portion and the actuator arm in horizontal and vertical direction increases. Thus, since a contact area between the molding portion and the actuator arm in the horizontal and vertical directions increases, a coupling force therebetween increases so that a stable operation of the actuator can be guaranteed.