A venetian blind includes a frame unit, a slat unit, a transmission unit, and an operating unit. The slat unit includes multiple drive axles and a plurality of slats each connected co-rotatably to the drive axles. The transmission unit includes two transmission rollers, a plurality of constant-force coil springs that are linked together and that are coupled to the drive axles, and a belt that cooperates with the constant-force coil springs to form a loop trained on the transmission rollers. The operating unit includes an inner control member connected to the belt, and an outer control member operable to drive the inner control member to move so as to drive rotation of the drive axles via the constant-force coil springs to rotate the slats.