Abstract: Ringing in the antenna, antenna bandwidth and the capability of radiating a clean pulse are important parameters for the design of ultra wide-band antenna .In this paper the FDTD (finite-difference time-domain) numerical method is used to study the relationship between the radiated waveform, the shape of aperture and the width of pulse under the condition of given source in the aperture. The radiated waveform in E-plane and H-plane for different aperture shape is obtained.