A head assembly of a golf club includes at least one various-shaped aiming mark formed in a centerline aligned with an impact line in a face of a head of a driver, an iron, or putter, or at the position opposite to the aiming mark about the centerline. Since the golfer aims when striking the golf ball, the habit of continuously keeping the golfer's aim to the head of a golf club is accustomed to the golfer such that the golfer naturally habituates the careful swing of a golf club.