Glass reinforce polyester (GRP) is the polymer composite which strength and stiffness change with various environmental factors. Temperature and humidity are two important factors in the manufacturing of GRP composites. Thisproject intend to determine the effect of temperature and humidity on the GRP laminates during its curing period. A series of specimens were prepared and cured under variious temperature and humidity. The specimens were then cut to the specific dimension to carry out the tensile test and flexural test.The results oobtained were then analysed and graphs were plotted. The graphwould show the relationship between the mechanical properties of GRP and the curing temperature and humidity. The conclusion of the experiment is thatthe mechanical properties of GRP lamination will decrease with an increase in temperature or humidity.