This paper presents a waveguide slit array antenna with high gain and narrow beamwidth. This antenna is formed by subwavelength slits surrounded with periodic grooves. The far field radiation characteristics of the improved antenna are investigated. Simulation results show that the gain of the antenna with a designed working frequency is improved by 11 dB and a strong beam can be obtained. The physical mechanism for the performance improvement is governed by the resonance excitation of surface electromagnetic waves and can be well described by the coherent superposition of power radiated from the grooves and central slits. (C) 2007 American Institute of Physics.