A chuck includes a body having an engaging groove for releasably receiving a bit. A sleeve is slideably mounted around the body. The body further includes an annular groove in an outer periphery thereof. The annular groove includes a positioning portion having an opening extending into the engaging groove. A positioning member is slideably received in the positioning portion and movable between a disengaged position disengaged from the bit and an engaged position. The positioning member in the engaged position has a portion extending through the opening into the engaging groove to engage with and retain the bit in the engaging groove. A retainer ring is mounted in the annular groove and clamps the positioning member in the engaged position.