An adapter structure for handheld electrical tools is disclosed, which includes an outer socket and an inner socket. The outer socket is provided with a shank at its bottom and provided therein with first teeth. The inner socket has an upper section, a middle section, and a lower section. The middle section is provided at its bottom with second teeth. The inner socket is fitted into the outer socket and limited by a crimp formed at a top edge of the outer socket, so that the inner socket will not be detached from the outer socket. When a selective tool, fitted at the upper section, is pressed to move downwardly, the first teeth can engage with the second teeth, so that the outer socket can drive the inner socket with the selective tool to rotate bolts or nuts under torque limitation for fastening operations.