Abstract: A new method of onsite measuring sphericity was proposed based on highprecision sphere grinding machine. The measuring basis and the automatic measuring instruments were setup based on the sphere grinding machine. The radius variation as well as corresponding longitude and latitude values of the measuring points evenly distributed over sphere were measured. The mathematical models of the spherical center coordinate and the least square radius were established based on the least square method. The least square sphere was then fitted by the measured values. The comparison charts of the measured sphere and the least square sphere were drawn. The onsite measurement of sphericity was achieved and the result shows the feasibility comparing with traditional micrometer based measuring method.