The diffraction characteristics of vortex beam through the circular aperture are studied. The purpose is to determine the number of topological charge of vortex beam based on the obtained diffraction patterns. The diffraction of vortex beam through a circular aperture is investigated theoretically and experimentally and the outcome can be achieved through experiments. The influence of the topological charge (including integer and fraction) of vortex beams on the diffractive pattern are analyzed in detail. It is shown that the topological charge corresponds with the light intensity distribution. The results may have potential application in determinating the topological charge of vortex beams, and the number of topological charge can be measured according to diffraction patterns.