A screwdriver and a ratchet mechanism thereof are disclosed. A ratchet mechanism includes: a main body base having blocking portions; a ratchet elastic sheet having abutment portions corresponding to the blocking portions; two blocking sheets retained on the blocking portions and abutted the abutment portions to be positioned between the blocking and abutment portions; a pinion having a gear portion between the blocking sheets to allow the gear portion and the blocking sheets to be adjusted between engagement/disengagement states; a positioning elastic sheet having an operating portion; a ratchet direction adjuster having contact members corresponding to the blocking sheets and an operated portion corresponding to the operating portion. The operating portion operates the operated portion, the contact members contact the blocking sheets and the blocking sheets being operated by the contact members to be in disengagement state with the gear portion to adjust the rotational direction of the gear portion.