Abstract: The near field properties of circular aperture antennas with Gaussian amplitude distribution are analyzed by the aperture field integration method in this paper. The axial near field and gain expressions of the unfocused and focused circular aperture antennas are deduced. The near-axis field distribution of circular aperture antennas is obtained by numeric integration method. The analysis shows that for the circular aperture antennas with Gaussian amplitude distribution, axial near field varies in an oscillatory manner and the near-field gain is related to both aperture distribution and the distance from the antenna. The near-field gain in the focus can achieve the gain in far field and the field intensity near the focus can be enhanced greatly through aperture focusing.