A new diameter measurering method of circular aperture based on Fresnel diffraction is proposed.Under the irradiation of parallel light,there is a special relationship between the diameter of peak contour in Fresnel diffraction light intensity distribution and the diameter of circular aperture to be measured,and then the diameter of the circular aperture can be measured according to the relationship.Simulations and experiments show that the accuracy of this method can reach up to submicro order for the measurement of the circular apertures with diameter of 5~ 10 mm.