The influence of circular aperture on the intensity of high-order harmonic generation (HHG) with intense femtosecond laser pulse was studied both experimentally and theoretically. The intensity variety of HHG with the diameter of circular aperture was observed in pulsed Ar gas. The result was discussed and interpreted in terms of the theory of Hankel transform. It is found that using the Gaussian beam truncated by an aperture could enhance the conversion efficiency of HHG at certain conditions.