A hexagonal wrench has a head. The head has an end surface, six equal side surfaces and at least one groove. Each side surface has a left edge segment and a right edge segment adjacent to the left edge segment of an adjacent side surface. The at least one groove is formed in at least one of the right edge segments and the left edge segments and extends to the end surface. Therefore, the head can engage securely an inner surface of a socket to rotate the socket even when the socket is worn or rounded.