A fast C-clamp comprises a generally C-shaped frame and a fixed jaw disposed in a front end of the frame. A fixed handle has an interior to receive a driving handle. The driving handle has an upper flange disposed on an upper end of the driving handle, a pivot hole and a rectangular hole formed on the upper portion of the driving handle. The driving handle and the fixed handle are fastened together. A holding seat is disposed in front of the fixed handle. A lower flange with a slot therein is disposed beneath the holding seat. A through hole is formed on the holding seat. The rear end of the frame is positioned in the slot. A round hole and a square hole are formed on the rear upper portion of the fixed handle. A shaft has a front end to connect a ball and a rear end to connect a holder. The ball is inserted in a rear portion of a movable jaw. A push plate with a circular hole thereon is disposed in the interior between a compression spring and the upper flange. A press plate has a center hole. The press plate is inserted in the square hole to be confined by a rear wall of the fixed handle. The shaft passes through the corresponding holes.