A transmission particularly for bicycles, which is capable of multi-speed gear-shifting by means of reverse rotation and comprises: a transmission axle rotatable in both positive and reverse directions, a speed-changing gear set, a speed-changing cam set, and a transmission seat. The speed-changing gear set comprises a plurality of transmission gears and speed-changing gears which are respectively engaging with the transmission gears. When the transmission axle rotates reversely, the speed-changing cam set can be driven in order to control and decide which transmission gear of the speed-changing gear set is to be engaged and driven by the transmission axle during positive rotation, so as to achieve the feature of multi-speed gear-shifting by means of reverse rotation.