A re-keyable cylinder lock includes a plug inserted into a lock housing and having a plurality of radial holes connected to a key hole. The lock housing has a first engaging element provided in an inner surface thereof. A limit unit has a second engaging element and is inserted into one of the radial holes located at a predetermined depth of the key hole. The first engaging element engages the second engaging element to limit the plug from rotating when a replacement key which is inserted into the key hole for re-keying does not reach a predetermined depth of the key hole. Incomplete re-keying can therefore be prevented.