A positioning device for a window covering includes a base, two clamping members, and an operating member. The base has a main recess and a cord bore, and the cord bore is communicated with main recess, and a cord is inserted into the cord bore and passes through the main recess. The clamping members are movably received in the main recess of the base, and the cord passes through a space between the clamping members. The operating member is connected to the base for rotation to drive the clamping members to move. While the operating member is turned, the clamping members will be moved toward each other to clamp the cord.