The mathematical modeling for evaluation of the sphericity error is proposed with minimum radial separation center. To obtain the minimum sphericity error from the form data, a geometric approximation technique was devised.The technique regarded the least square sphere center as the initial center of the concentric spheres containing all measurement points,and then the center was moved gradually to reduce the radial separation till the minimum radial separation center was got where the constructed concentric spheres conformed to the minimum zone condition.The method was modeled firstly,then the geometric approximation process was analyzed,and finally,the software for data processing was programmed.As evaluation example,five steel balls were measured and the measurement data were processed with the developed program.The average iteration times of the approximation technique is 4.2,and on average the obtained sphericity error is 0.529 μm smaller than the least square solution,with accuracy increased by 7.696%.