A crank structure for a curtain has a base, a rolling wheel set, an engagement set and a crank. The base has a bottom plate and a limiting frame. The rolling wheel set is disposed in the storing space of the base and has two rolling wheels. Each central aperture is jacketed onto the central shaft of the storing space. The two rolling wheels is further respectively provided with a braking gear and an inclined ring. Each inclined ring has a plurality of engagement ribs. The engagement set disposed in the assembling portion of the base and has a socket, a supporting wheel and a bottom member. An engaging pattern is provided at a periphery of the supporting wheel, a passive gear is provided at the supporting wheel, and a shaft is provide at a center position of the supporting wheel.