A rekeyable lock cylinder comprises a plurality of first rack components, a plurality of second rack components and a guide bar having a first surface engaged with the first rack components. The second rack components are movable via contacting against a first matched key which is inserted in a first direction. The guide bar has a second surface, a first side, a second side, and a tool-receiving portion recessed into the first side. The tool-receiving portion is parallel to the first direction, and the first rack components are actuated by the guide bar via acting force applied by the rekeying tool accommodated in the tool-receiving portion therefore enabling the first rack components to move in a transverse direction relative to the first direction and disengaging the first rack components from the second rack components.