An actuator assembly usable un a hard disk drive includes an actuator arm having an arm hole to improve dynamic characteristics. The actuator arm of the actuator assembly has an arm hole having first, second, and third extended portions, and two members positioned at both sides of the arm hole. The first extended portion is formed at an edge positioned at a leading end of the arm hole so that each of leading ends of the two members has a first reduced width, the second extended portions are formed at edges positioned at a rear end of the arm hole so that each of rear ends of the two members has a second reduced width, and the third extended portions are formed at both sides of a middle portion of the arm hole so that each of middle portions of the two members has a third reduced width. The third width is smaller than the second width and larger than the first width, so that a displacement of a read/write head in a major resonance mode is minimized, a bandwidth for position control of the read/write head is widened, and a position error signal is decreased when the hard disk drive operates.