A theoretical model of the sphere far-field interference was given. When two coherent plane waves are irradiating the smooth sphere at any included angle, the reflected waves interfere in the far-field. A formula on optical path difference of reflected waves is derived in the far field. The sphere diameter can be accurately measured with far field interference and a formula on measuring the diameter is given. The measurement system contains spectrometer, CCD camera, helium-neon laser, optical platform and computer. The method is given to measure the diameter of the sphere with original laser beam. The measured results of diameter by far-field interference are compared with that by the Abbe comparator, the relative difference of which is 0.2%. The measurement error generated by Gaussian beam is 5×10－5mm and can be negligible. This measurement method can achieve self-camera lens distortion correction. It shows that the theory and method of measurement is accurate. Gaussian laser beams is possible to measure the diameter of the sphere.