Core-sheath structured electrospun fibers with styrene-butadiene-styrene (SBS) block copolymer as a rubbery core and polyacrylonitrile (PAN) as a hard sheath were prepared by coaxial electrospinning, and used to improve the toughness of epoxy resin. The surface of the fibers was aminated by reacting PAN with diethylenetriamine to improve the interfacial interaction between the fibers and epoxy. Scanning and transmission electron microscopies confirm the core-sheath structure of the PAN/SBS fibers. The Charpy impact energy is increased by the addition of electrospun fibers. When the content of aminated core-sheath fibers is 4 wt %, the Charpy impact energy is increased by 150%. Dynamic mechanical analysis shows that the glass transition temperature of epoxy is not decreased by the addition of core-sheath fibers. The high impact resistance is attributed to the rubbery core of the fibers that can absorb and dissipate impact energy, and the chemical bonding between the fibers and epoxy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41119.