Abstract: Starting from the Collins formula,focal shifts of flattened Gaussian beams(FGBs)passing through an aperture lens are studied and illustr ated with detailed numerical examples.It is shown that focal shifts of apertured FGBs not only depend on the beam order N,Fresnel number Fw associated with the beam,but also depend on the Fresnel number Fa associated with the aperture.In comparison with the unapertured case,there still exist fo cal shifts even for the case of the large Fresnel number Fw.