A cord guiding device for a window blind includes a wheel holder, a guiding wheel and a cord-blocking unit. The guiding wheel is rotatably mounted on the wheel holder, and includes two protruding flanges that are spaced apart from each other, and has a wheel surface that is defined and disposed between the protruding flanges and that is adapted for a cord of the window blind to wind thereon. The cord-blocking unit is mounted on the wheel holder, and has a blocking surface that faces and is disposed proximate to the wheel surface of the guiding wheel for preventing the cord from slipping off the wheel surface and out from the protruding flanges.