The effects of the fiber reinforcement of a novel bioabsorbable chitin-fiber-reinforced poly(epsilon-caprolactone) (PCL) composite were improved by irradiation treatment. The tensile strength and tensile modulus of the treated specimens were enhanced with respect to those of the untreated specimens. An increase in the fiber content (C-f) resulted in an increase in this enhancement tendency until C-f was 45%. A further increase in C-f increased the tensile modulus but decreased the strength. The flexural strength and flexural modulus were increased for the irradiation-treated specimens in the same way as the tensile test. The microstructure of the tensile fracture showed an improvement in interfacial bonding for the irradiated specimens. The glass-transition temperature (T-g) of the composite increased with an increase in C-f for the irradiation-treated specimens, but there was no change in T-g for the untreated specimens with various values of C-f This indicated that, for the composites with irradiation treatment, the fiber intensively affected the molecular segmental motion of PCL and thereby enhanced the interfacial interaction between the matrices and fibers. The same slope of the storage modulus (G') versus the loss modulus (G") for the irradiated specimens suggested an increase in the compatibility of the composite in comparison with the decrease in the slope with increasing C-f for the untreated specimens. All this demonstrated that there was some interfacial reaction between the fiber and matrix that resulted in the presence of an interfacial phase and improved the mechanical properties of the materials.