In recent years Reflectarray Antennas have received considerable attention due to their unique capabilities. However, due to their large size, analyzing the performance of these Antennas using traditional full wave finite-difference and finite-element techniques requires considerable computational resources. In this paper, we present an efficient method to accurately analyze these class of Antennas which considerably reduces the computational burden. First, the radiation properties of the unit cells of the Antennas are obtained by applying periodic boundary conditions, which corresponds to an infinite array. Then, a phase-only algorithm is used to obtain required phase shift on the Antenna surface. Next, different unit cells are used to achieve the required phase on the Antenna surface. Lastly, to confirm the validity of our approach, each designed Antenna is simulated by full wave method using method of moment and the full wave simulation results of co and cross polarization levels are compared with those obtained using the proposed formula. There is excellent agreement between the co- and cross polarization levels obtained by proposed approach and those obtained using full wave simulation.