Abstract: A locking mechanism installed in an electric hand tool to automatically lock its lower output shaft including a toothed annular member fixedly mounted inside the shell of the electric hand tool, a holder base mounted inside the annular member, two locking members pivotably mounted in respective holes in the holder base and forced by a spring plate into engagement with the toothed annular member, and a transmission member having a bottom center shaft fitted into the axial center through hole of the holder base, wherein the locking members are forced into engagement with the toothed annular member to prohibit rotary motion of the transmission member relative to the holder base when electricity is cut off; one locking member is forced to disengage from the toothed annular member, when the transmission member is turned in one direction after the connection of electricity to the electric hand drill.