Glass fibre-reinforced polymer have been used as an alternative to steel in concrete due to high strength-to-weight ratio, high stiffness-to-weight ratio, and corrosion and fatigue resistance. The main environmental factors for the deterioration of GFRP sandwich composites are temperature, sunshine, water/moisture, alkalinity and load. In this an experimental investigation has been carried out to study combined effect of chosen parameters moisture and temperature & also to study the rate & magnitude of damage of GFRP sandwich structure composites under hygrothermal loading conditions. The sandwich structure composed of E-glass fibre, polystyrene (thermocol core) and epoxy resin were subjected to three point bending test to assess bending strength. Two thermocol cores with different thickness (8mm and 16mm) were investigated. It has been observed that the sandwich structure with higher core thickness withstand a higher bending load show less flexural stress and flexure modulus when compared with low core thickness. Higher percent bending pre-loads lead to higher degradation in maximum flexure load as well as flexure modulus and flexure stress.