In this work, an X-band flat-gain transmitarray (FGT) antenna is proposed and investigated. The designed FGT is able to provide reliability, compactness and broadband characteristics with high efficiency. The transmitarray consists of multilayer stacked patch cells, which can transmit electromagnetic waves from one side and then provide proper phase changes controlled by variable lengths of microstrip lines for the outward waves. It owns the feature that the outward wave is of same polarization as the inward wave. The proposed FGT is composed of 10×10 elements covering an area of 150 mm×150mm implemented at 10 GHz. The FGT is center-fed by a standard pyramidal horn. Preliminary results show that the proposed FGT provides 61% aperture efficiency at 9.4 GHz with 1-dB gain bandwidth of 9.2–10.9 GHz. An impedance bandwidth is obtained from 8.7 GHz to 12.2 GHz.