A ratchet cylinder for a bicycle hub is provided with a cylinder member, a plurality of ratchet teeth, a plurality of springs and a C-ring. The C-ring circles around the connecting end of the cylinder member and engages in the grooves of the respective ratchet teeth, so as to retain the ratchet teeth in the receiving grooves and allow the ratchet teeth to pivot within a limited angle. The springs are disposed between the ratchet teeth and the ratchet cylinder to keep pushing the respective ratchet teeth of the hub shell into an engagement position with the teeth of the hub shell, so that the ratchet teeth can be surely engaged with the teeth.