A planar three DOF hybrid mechanism is proposed based on the traditional planar three-limb parallel mechanism. The rotatablility of the hybrid mechanism is investigated by the crank condition. The theorem of the assemblability and rotatability criterion are extended and these criteria are applied to detailly classfy the crank types for the hybrid mechanism. The crank conditions of the hybrid mechanism are deduced according to link constraint dimensions. The conclusions are helpful for quickly judging the existence and the type of crank situations. It is the base to further research about the hybrid mechanism.