The reaction at narrow holes method (RANH method) has been proposed for fabricating fiber reinforced metal (FRM), such as an intermetallic compound fiber / metal matrix composite. This study clarifies a microstructure at a fiber / metal matrix interface of FRM fabricated by using a combination of pure-copper and pure-aluminum in the RANH method. Pure-aluminum fiber was inserted into a narrow hole drilled in the copper matrix. The assembly comprising the pure-aluminum fiber and the pure-copper matrix was heated to a temperature greater than eutectic temperature of the copper-aluminum binary alloy. A molten aluminum reacted with copper to form an annular reacted region consisting of g1 intermetallic compound in a single phase near the edge of the narrow hole. The g1 intermetallic compound has very high hardness on the order of 800-900 HV. The annular reacted region may have a high tensile strength and may work as a reinforcing metal fiber in FRM.