The inter laminar stress distributions, obtained by three dimensional FEM, were discussed in order to explain the initial failure which was observed in the four point flexural test on the hybrid FRP made of unsaturated polyester resin reinforced by carbon and glass fibers. In three dimensional FEM, the thermal residual stresses due to the curing process were taken into consideration, too. It was found that the initial failure loads predicted by applying Hoffman's law well agreed with the experimental results.