The surface free energy and its dispersive, polar, electron acceptor and electron donor components of RDX were determined by the Washburn equation with thin-layer wicking technique. The diiodomethane and 1-bromonaphthalene, which are neutral probe liquids, were used to determine the dispersive component of RDX. And the ethylene glycol and formamide, which are polar probe liquids, were used to determine the polar components of RDX. The results show that the surface free energy and its dispersive, polar, electron acceptor components increase with the increasing of particle size. And the electron donor component takes the opposite trend. Films of RDX/HTPB/IPDI with 15% mass content of RDX with different particle size were prepared. The effect of particle size and surface free energy of RDX on the mechanical properties of HTPB propellant were studied. The results show that the maximum stress well-regulated varies with the interface adhesion energy between HTPB/IPDI binder system and every RDX particle, which is y=0.5242+0.6973exp(-1.616w⁰_adh). The maximum stress of RDX/HTPB/IPDI system decrease with the increasing of the interface free energy of every RDX particle.