[Purpose] This study investigated the difference screwdriver handle size has on forearm muscle activities and wrist motion during screw-driving work. [Subjects] Fourteen males aged 20–30 years, were recruited. [Methods] We measured the forearm muscle activities using a MP 150 System and wrist motion using a 3D motion system. The subjects performed screw-driving tasks with different screwdriver handle sizes. [Results] The wrist flexion and ulnar deviation angles and flexor carpi ulnaris muscle activity significantly increased and wrist extension angle significantly decreased during screwdriver work with a thin handle. [Conclusion] We consider that industrial workers should perform screw-driving work using a screwdriver with a thick handle.