A resistance device includes a base having a channel, a friction member provided in the base, and a moving member adjacent to the friction member, wherein the moving member is located in the channel and is movable therein. A window covering applied with the resistance device includes a covering material and a lifting cord concurrently movable with the covering material. When the covering material is being retracted or extended, the lifting cord moves in two opposite directions, respectively. The lifting cord passes through the resistance device. When the lifting cord moves in either direction, it drives the moving member to move in the same direction as itself. The friction member provides different resistances to the lifting cord which moves in different directions.