A low-profile high efficiency transmitarray antenna (TA) using an optimized phase compensation surface (PCS) and PEC sidewalls is proposed in this paper. Generally, TAs are composed of a source antenna and a planar PCS, and the PCS makes the phase of transmitted wave through the PCS be in-phase, resulting in a highly directive beam. The unit cells of a PCS cannot have a 100% transmittance at all transmission phases. Since there may be combinations of PCS that make various in-phases, the transmit efficiency of the TA can be improved. Also, the four PEC sidewalls are employed to maximize the amount of power reaching the PCS from the source antenna. As a result, the 100% spillover efficiency is achieved without diminishing taper efficiency. To verify its feasibility, the square TA was designed and measured at 5.8 GHz and a high aperture efficiency of 54% was obtained through the above design methodology.