A control device for coupling with a shaft of a window blind and controlling rotation of the shaft includes first and second ratchet seats, a spool assembly operable to rotate in unwinding and winding directions, a tubular case, an output unit coupled with the shaft, and a transmission unit operable to be shifted between a transmitting state to have a torque transmitted to the output unit, and a non-transmitting state to interrupt the torque transmission. Each ratchet seat has a surrounding groove and teeth. Balls are rollably engaged in the surrounding groove and the spool assembly for permitting rotation of the first ratchet seat with the spool assembly in the unwinding direction, and not to rotate the first ratchet seat in the winding direction.