The effect of electron beam irradiation on silk fibroin (SF) fiber-reinforced poly(epsilon-caprolactone) (PCL) composite was investigated by mechanical test, scanning electron microscope, dynamic mechanical thermal analysis, electron paramagnetic resonance (EPR) analysis, and Fourier transform infrared (FTIR) analysis. The results indicate that electron beam irradiation can affect the static and dynamic mechanical properties of PCL and SF/PCL composite, depending on the irradiation dose. PCL shows maximum strength and modulus at the irradiation dose of 200 kGy, and the SF/PCL composite with 45% fiber content exhibits maximum strength and modulus at the doses of 150 kGy. EPR analysis shows that during irradiation, both PCL and SF fiber can produce free radicals. Some transformations or reactions of radicals may take place between PCL and SF fiber and enhance the interfacial interaction. The microstructures of tensile fracture also show an improvement in interfacial interaction between fiber and matrix after irradiation. FTIR analysis shows that chain scissions also occur in both PCL and SF fiber during irradiation. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1063-1069, 2009