In this paper, analysis, design and simulation of a planar 8 × 8 slot array antenna with low sidelobe level (SLL) in H-plane which is implemented on the slow wave structure (SWS) at 15 GHz is presented. In order to show the method of reducing the antenna size, the dispersion of the slow wave structure is discussed. By implementing the SWS in the slot array antenna, 30 % size reduction is achieved. The design procedure of the antenna array is also explained in this paper. The designed antenna is finally simulated which shows that the SLL and gain are about -25dB and 18.3dBi respectively.