A rekeyable lock cylinder comprises a cylinder body and a plug assembly disposed within the cylinder body. The plug assembly comprises a plug body, a plurality of assembled pins and a position block. The plug body has a longitudinal axis, a transverse axis vertical to the longitudinal axis, a keyhole and a first through hole. Each of the assembled pins is movably disposed in the plug body and comprises a first rack component and a second rack component selectively engaging with the first rack component. The position block disposed at the plug body has a plurality of pin runners used for disposing the first rack components, a second through hole corresponding to the first through hole and a tool-contacting surface located within the second through hole. The position block can move along the transverse axis-direction of the plug body to make each of the first rack components reengage with each of the second rack components.