Abstract The traditional method for space plane circle fitting includes two steps. The first step is to determine the space plane equation and transform the coordinate system into another coordinate system where z axis is perpendicular to the space plane. Secondly, the plane equation of a circle is fitted according to the x and y coordinates. Considering the space plane circle is essentially constructed by space plane and spherical plane which is intersected, an approach which takes a space plane equation and spherical equation as the constraint condition has been put forward, which includes directly calculating space plane circle observation equation and the corresponding calculation formula. The measured data of a certain space circular object is taken as an example to verify practical application of the method.
