It has been shown that the direct use of tension and compression stress strain models underestimates the flexural strength of epoxy resin materials. The value of the flexural over-strength factor which is the ratio of the flexural strength to the strength obtained from tension and compression stress strain models depends on stress gradient, size and loading system. Fifty-two small-sized three point bending samples of epoxy resin Epon E863 and eight small-sized three point bending PR520 were tested at room temperature. All the samples had a groove in the middle of the beam. The Method of Moments and the Least Square Analysis Method were used to determine the Weibull parameters. Different common estimators for probability of fracture were used to estimate the flexural over-strength factor. The two-parameter Weibull model overestimates the flexural over-strength factor for epoxy resin materials.