Materials found in earth are nonrenewable and scares. As the need of human being increases exponentially, new and alternate materials and manufacturing methods are developed and utilized extensively. Manufacturing of composite material has been an extensive area of research as they have high strength-to weight ratio that are equivalent or superior to many metallic materials. This paper describes the preparation of woven fabric or chopped strand fiberglass Reinforced Polymer Composite (GFRP) water reservoir with different fiber mat material, orientation, unsaturated polyester resin and different ratio between resin and woven fabric or chopped strand fiberglass. The purpose of this paper is to investigate the influence of the process parameters on the mechanical properties of GFRP composite using Taguchi experimental design in combination with Orthogonal array (OA), seven factors ( one has two levels and the rest have three levels) and L18 (18 experiments were conduct).The predicted and the observed values are close to each other, then the used model is adequate for describing the effect of Parameters on quality characteristics and if there is a large difference in observed values and predicted values then the used model is not adequate. The conclusion revealed that number of layers of woven or chopped strand fiberglass, types of fiberglass , different ratio between resin and fiberglass and the mixing time of polyester and calcium carbonates were the most influential factor on the mechanical properties ,percent contribution 70.60%,26.71%,1.88% and 0.51% respectively. It is observed that the optimum properties were obtained at number of layers of woven or chopped, types of fiberglass, ratio between resin/unsaturated polyester and fiberglass (3:1 is the best) and the mixing time of polyester and calcium carbonates (A2B3C2D3E3F3G3 optimum level of the parameters).