A device for adjusting a slat of a window blind including a housing, in which a plurality of frames, a rod, a plurality of transmission members, and a plurality of turning members are provided. The rod is meshed with a first rack of each transmission member, and the turning members are respectively meshed with a second rack of each transmission member. The first rack has a plurality of teeth in a line, and the first and the last teeth are shorter than the rest teeth. Furthermore, the first and the last teeth are designed for ratchet, so that the teeth of the rod will slide on and over the first or the last teeth when the user is overturning the rod to protect the teeth of the first rack of the transmission member.