A device of folding a seat for a vehicle may include a seat cushion disposed on a bottom of a vehicle body and rotatable by a rotation mechanism and provided with a first position fixing mechanism, a seat back rotatably disposed at a rear of the seat cushion and provided with a second position fixing mechanism, a headrest rotatably disposed at an upper portion of the seat back and fixed in position by a third position fixing mechanism disposed in the seat back, and an operation mechanism disposed in the seat back and unlocking the first position fixing mechanism to unlock the second position fixing mechanism and the third position fixing mechanism at a time of rotating the seat cushion so as to rotate the seat back and the headrest.