Fiber reinforced plastic (FRP) composites are used in adverse environmental conditions i.e., variation of temperature, humidity, seawater, acidic water, chemicals, organic fuels, etc. It is important to investigate the crushed and interlaminar shear strength (ILSS) behavior of these composite materials under adverse conditions. This study shows the effect of temperatures, seawater and distilled water on crushed and ILSS of glass fiber reinforced polyester composite panels made by a recently developed process known as rubber pressure molding (RPM) technique over a range of temperature from 0 to 150°C. The fiber volume fraction in the composite varies from 30 to 60%. The RPM technique is based on the matching die set, where the die is made of hard metal like steel and the punch from flexible rubber like materials. The use of flexible rubber punch helps to intensify and uniformly redistribute pressure (both operating pressure and developed hydrostatic pressure due to the flexible rubber punch under compression) on the surface of the complex shaped product. Natural rubber was used to prepare a rubber punch in this investigation. For performance evaluation of FRP composites made by RPM technique, FRP composites were also made by the conventional method and tested at the same temperatures. It is observed that the crushed and ILSS of FRP composites decreases towards higher extreme of the temperature range selected. FRP composites made by RPM technique show higher crushed and ILSS over the temperature range of 0–150°C compared to the FRP composites made by conventional process. Again crushed and ILSS increases with increasing fiber volume% in the composites made by both techniques. In addition to these results the crushed and ILSS decrease with dipping time both in distilled and seawater. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.