This paper is principally concerned with the form error measurement and evaluation of the spherical surface after precision machining. According to the minimum zone condition, the definition of sphericity error is first analyzed. To obtain the minimum sphericity error from the form data, an approximations technique is devised. First the method is modeled, then the process of approximations is constructed, and finally software for data processing is programmed. Six steel bolls of φ50 are measured on a coordinate measurement machine (CMM) in the Measurement experiment. The experiment results indicate that the geometric approximations technique can give an accurate solution with high efficiency.