Abstract: A fluid model is used to describe Cerenkov free electron lasers with annular relativistic electron beams contained in dielectric-loaded cylindrical waveguides. The dispersion relation is derived, and the growth rate is given for the Cerenkov instability. Based on the saturation mechanism, the output power is estimated. Results show that the gap between the beam and the dielectric can degrade the beam-mode coupling, and the efficiency and output power for an annular beam may be higher than that for a solid beam.