An operating device for rotating a winding roller of a window blind, includes an upright rotating shaft rotatable to cause rotation of the winding roller and having thread segments, a sleeve shell surrounding the rotating shaft, upper and lower coupling members anchored on the sleeve shell and each having grip ends and cammed surfaces confronting the thread segments and the sleeve shell, and an actuating member having a camming surfaces disposed to be moved upward or downward to make an upper or lower friction engagements between the upper coupling member and rotating shaft, and between the lower coupling member and the rotating shaft for rotating the rotating shaft.