A ratchet mechanism of a bicycle hub includes a ring fixed in the hub. The ring has a tapered surface and a flange is formed in the inner periphery of the ring. A driving unit is located in the hub and has multiple first ratchet teeth extending axially therefrom. A passive unit is located in the tapered surface and corresponding to the driving unit. The passive unit has a passive seat which has a tapered outer face matched with the tapered surface. The passive seat further has a stop portion which is stopped by the flange in one direction. The passive seat has multiple second ratchet teeth engaged with the first ratchet teeth. A magnetic unit is located between the hub and the passive set, and two magnetic members which are magnetically repulsive to each other so as to linearly move the passive seat toward the driving portion.