In order to study the influence of explosive crystal particle size on mechanical properties and interfacial enhancement of polymer bonded explosive （PBX）， HMX-based PBXs were prepared by using four kinds of HMX with different particle sizes （160 µm，60 µm，25 µm and 150 nm） as main explosive. The fluorine resin and neutral polymeric bonding agent were used as binder and interface enhancement agent， respectively. The compressive stress-strain test and Brazilian test were performed to obtain the compressive and tensile mechanical properties of 8 types of PBXs at room temperature （20 ℃） and high temperature （60 ℃）， respectively. The storage modulus and mechanical loss factor were obtained using the three-point bending mode of dynamic mechanical analysis. The cross-sections of PBXs were characterized by scanning electron microscopy. Results show that both the compressive and tensile mechanical strength of explosive increase with the decreasing of HMX particle size， in which the nano-HMX based PBX （PBX-nano） shows the highest strength. The compressive strength and tensile strength of PBX-nano are 61.3 MPa and 5.7 MPa， increasing by 73.1% and 63.5% from PBX-L， respectively. By adding the bonding agent， the compressive mechanical strength and tensile mechanical strength for all the PBXs are significantly improved， especially the PBX-nano. The tensile strength of PBX-nano-M at 20 ℃ and 60 ℃ are up to 10.4 MPa and 5.8 MPa， which are 82.6% and 101.4% higher than that of PBX-nano， respectively. When the average particle size of HMX decreases from 100 µm to 100 nm， the greater the fracture energy required for interface debonding/damage and even fracture of explosive， the greater the increase in tensile mechanical strength.