The reaction layer formed at interface between matrix and fiber has significant effects on the mechanical properties and behaviors of deformation in FRM. In this study, the mechanical properties and interfacial behaviors according to surface finishing on the fibers and according to heat treatment in FRM were investigated. FRM was fibricated by diffusion bonding method. In W/Al alloy composite and W/Al composite, W of which was coated with WO₃, the heat treatment was carried out thermal cycling method from 373K to 673K. In W/Al composite, W of which was coated with WO₃, growth of interface layer was hardly occured in spite of the increasing various thermal cycles. It was exhibited that oxidized W/Al composite were higher strength than non-oxidezed W/Al composite with the increasing thermal cycles. The compounds of fiber/matrix interface were analyzed into WAl_(12), WAl_5 and AlWO₃, respectivly. Therfore the interfacial compounds of fiber/matrix seriously affected the mechanical properties and behaviors of deformation in FRM.