The mechanical properties of a polymer bonded explosive based on HMX thermally aged under 75 ℃ were investigated. The variation of the density of the samples is less than 0.003 g·cm^-3 before and after thermal aging. The results of the mechanical testing show that the compressive and tensile properties changed reversely after thermally treatment,that is,the compressive properties were dereased while the tensile properties were increased. Combined using the computer tomography (μCT),X-ray photoelectron spectrometer (XPS) and ultrasonic microscope,it shows that the density decrease of the PBX maybe resulted from the volatilization of the lower-melting-point indigent of the formulation,that is the reason for the decrease of the compressive strengh. Contrarily,the interface bonding between the HMX crystals and the binder has been improved throug the thermally aging such that the tensile mechanical properties as well as the tensile creep performance have been improved correspondly.