Polymer composite structures are usually subjected to large flexural loadings during their life-span of the structures, so the flexural behavior of these structures and their constituents in different environmental conditions are critical to their use. A novel analytical approach for epoxy resin semibrittle materials with strain softening model in tension and compression has been developed to investigate the flexural behavior of these materials. The value of the flexural overstrength 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 and it has already been evaluated at the laboratory condition. The mechanical properties of epoxy resin materials are sensitive to environmental effects at which they are loaded. The influence of temperature 60°C and humidity 90% Rh on tension, compression and flexural behavior of epoxy resin polymeric materials PRI and PR 520 have been investigated. Digital image correlation system was used for material characterization. The effects of heat and humidity on softening localization in flexural response were considered. The influence of heat and humidity on the flexural over-strength factor was evaluated.