The effect of nitramine particle size on the combustion behavior of inert binder based propellants has been extensively studied for RDX and HMX, but not CL-20. Although materials such as RDX and HMX are useful for particular combustion applications, CL-20 has a greater potential to improve the oxygen balance and energy density of a propellant. The current work investigates the effect of CL-20 particle size on the combustion of CL-20/HTPB propellants down to submicrometer sizes. An influence of particle size on the burning rate and combustion mechanism is reported. The 30 micrometer formulation burning rate data showed evidence of convective burning specifically at higher pressures, but the pressure dependence was comparable to neat CL-20 at pressures below 8 MPa. A change in the combustion mechanism of the submicrometer formulation as a function of pressure was determined to be a result of the interaction of the propellant flame and the combustion residue. Data suggested that at low pressures diffusion in terms of active cooling was dominant for the submicrometer formulation. Higher pressure data for both the submicrometer and 3 micrometer formulations suggest the degree of active cooling is decreased as the burning rate pressure exponent is near 0.5 for both propellants. The indirect evidence for the presence of a melt layer for CL-20 propellants is discussed.