An equivalent variable aperture method for mode field radius measurement of aingle mode fibers is proposed. It involves measurement of the integrated far-field distribution with a circular aperture moved axially in the far-field of the fiber. In sddition, calculation method of the mode field radius, which is also suitable for non-Gaussian field dis-tributions, is derived by using Petermann s new definition of mode field radius. Experiments have shown that this method is simple and reliable. The typical deviation of repeted measurements of the mode field radius of a given fiber with, different ends is less than 0.04μm.