A Cerenkov free-proton laser is proposed. Analyses shows that the interaction length of a Cerenkov free-proton laser (CFPL) may greatly exceed that of a Cerenkov free-electron laser (CFEL). The grain of a CFPL can thus be expected to be higher than that of a CFEL. In addition the energy loss in a CFPL due to bremsstrahlung is much smaller than that in a CFEL, so the requirement on the beam quality can be reduced.