The purpose of the present work is to reveal the behavior of two kinds of different matrix resins in composite materials through the experimental results of mechanical and fracture properties and observation of macro and micro fracture surfaces. The composite specimens are made of ductile or brittle resin and continuous glass fiber mats with various fiber contents. The elastic modulus and the tensile strength of these two resins are nearly of the same values, while the elongation is different. The results obtained are as follows: (1) The tensile strength of the ductile matrix resin composites is higher by 20~30% than that of the brittle matrix resin composites. The values of both composites with fiber content 20 wt%, however, are lower than those of the matrix resins. These results can be understood based on the observations of macro fracture processes and fracture surfaces. (2) The fracture toughness Kin at crack growth initiation increases with increasing fiber content for both composites. The values of Kin for the ductile matrix resin composites with fiber contents 0 wt% and 20 wt% are higher than those of the brittle ones. For the composites with fiber contents 40 wt% and 60 wt%, the values of Kin for both composites scatter randomly. © 1994, The Society of Materials Science, Japan. All rights reserved.