Polymer based composites due to special features such as high tensile strength and compressive strength, good fatigue and corrosion resistance are a good alternative to other materials in various industries. Composite structure properties are function of its components; their distribution, reaction and interface and so on. In general, composites are composed of the matrix and reinforcement. The responsibility of matrix is holding reinforcements together with specific orientation, protection against environmental conditions, abrasion reinforcement on each other and load transmission to the reinforcement. There are various processes to manufacture parts such as resin infusion, hand lay-up, Pultrusion, spray-up and filament winding. In resin infusion process, the dry fibers are compressed in the mold, before impregnated with a liquid thermoset matrix. The process is divided into various methods, one of the most common method is vacuum infusion process (VIP) that the vacuum pressure is used as driving force to reach liquid thermoset resin into the dry fiber layers. VIP needs to the low viscosity resin for completion of the infusion process and fully impregnated fibers. Hence Rheological modeling leads to better understanding of the resin effect on manufacturing process. In this study, a rheological model is developed to predict the effects of epoxy resin in isothermal conditions that had good agreement with experimental results. The effect of changing manufacturing process from hand lay-up to VIP on the mechanical properties of composites with two types of unidirectional fiber reinforcements and two types of layers orientation has been studied theoretically and experimentally. The results show that changing the method of manufacturing from hand lay-up to VIP, increases tensile strength and modulus to more than 40 percent, shear strength and modulus to more than 69 percent and impact strength to over 60 percent. The best theoretical models that predict the tensile and shear properties of the VIP samples precisely, were Chamis and Halphin-Tsai models and best models for hand lay-up samples were Rule of Mixtures (ROM) and Modified Rule of Mixture (MROM) models. Keywords: Composite, Vacuum infusion, Hand lay-up, Unidirectional fiber reinforcement, Mechanical properties