Composite materials are the largely used engineering materials in aerospace and automobile industries due to their high specific strength and high specific stiffness. The properties of the composites are very much important to design and develop the machine parts. They vary with fiber content, fiber properties, matrix properties, and type of manufacturing process. Number of experiments are required to obtain the properties and the best combination of fibers and matrices. However, several analytical methods are available to find the properties of the composite to avoid the number of experiments. In the present study, the properties of CFRP, GFRP, Amino-functional multi-walled carbon nanotubes (CNT) added CFRP, CNT added GFRP composites have been calculated by using the properties of fiber, interphase between fiber and matrix, matrix, and CNT. The properties of CNT added epoxy are obtained using Halpin-Tsai equation in first stage, and in the second stage, the properties of interphase are calculated using the properties of CNT added epoxy and fiber properties. The third stage, the properties of CFRP and GFRP are calculated using three phase constitutive model by considering the properties of fiber, interphase, and CNT added matrix. The properties are calculated at fiber diameters: 8 μm and 14 μm while varying the fiber volume fraction (%): 0 to 70%, interphase thickness: 50 nm to 500 nm, weight fraction of CNT (%) added in epoxy: 0 to 5%. The addition of CNT has improved the elastic properties of CFRP and GFRP. The elastic properties of the composites are improved significantly with increase in the interphase thickness.