Effect of particle size and types of nitramine on combustion performance of energetic thermoplastic elastomer (ETPE) gun propellants based on BAMO-AMMO were studied by closed bomb test. Results show that the pressure exponent of ETPE gun propellant is about 1.2 under 50-260 MPa,while the pressure exponent turns down from 1.2 to 1 as the pressure range changes from 75-175 MPa to 175-260 MPa. Burning rate and pressure exponent of the ETPE gun propellant increases with increasing of the patical size of RDX. It is also found that the grain size matching of RDX or mixed oxidizer containing RDX and HMX can increase the burning rate of the ETPE gun propellant and unconspicuously decrease the pressure exponent at 75-260 MPa. The sample of ETPE gun propellant using HMX as oxidizer has lower burning rate and higher pressure exponent than the sample of ETPE gun propellant using RDX as oxidizer.