A single ridged wave-guide slot phased antenna (SRWSPA) with a photonic band-gap structure which is alternately arranged on the antenna is presented. Another antenna with identical dimensions on a metal ground plane is also included, so that a direct performance comparison can be made. Measured data shows an obvious improvement of radiation pattern, a reduction of coupling coefficient between elements, a significant reduction of back lobe and an improvement of scanning performance. Meanwhile it also proves that the structure is more effective to suppress the surface wave than the choke ring. This is significant in improving the radiation performance of the single ridged wave-guide slot phased antenna.