Abstract: In this paper a new UWB horn antennas based on H-guide was proposed to resolve the problem that the general ultra-wide-band(UWB) antennas(i.e: TEM horn) have a large leakage in side. The possibility of the H-guide used as the UWB antennas and the rules of the parameters selected in theory is studied firstly, then the FDTD(finite-difference time-domain) numerical method is used to calculate the transformed characteristics in time-domain when the H-guide is excited by the coaxial probe and the radiation from an open-ended H-guide and it's horn. A conclusion can be gained that the H-guide can be used as the UWB antennas and the radiating elements to antenna arrays being small leakage in side.