Based on the accurate expression of light intensity at the transverse plane and the rigorous diffraction formulae, the characteristics of axial light intensity of Gaussian beam diffracted by a wavelength-order circular aperture are investigated. The results show that the axial light intensity characteristics of Gaussian diffracted beam are decided by the initial Gaussian half width w0 and the radius of wavelength-order circular aperture R. As to the Gaussian diffracted beams with w0/R≥1, the number and location of extrema of axial light intensity are determined by 2R/λ, and the maximum axial light intensity must be located at N=R2/(λz)=1. Whether the apex and vale of extrema are distinct or not rest with the ratio of w0/R. The larger the ratio, the more distinct the apex and vale are. When the ratio of w0/R is large enough, it trend to the case of planar incident wave. While for the Gaussian diffracted beams with w0/R<1, the axial light intensity exists specifically evolutive law. The number of extrema is reduced step by step and finally disappeared with the initial Gaussian half width diminishing.