Abstract： The measuring circle function of coordinate measuring machine (CMM) is studied and a new uncertainty evaluation model is established in the paper based on error ellipse theory and Monte Carlo method. The uncertainty of the sample points is expressed in the error ellipse. The paper uses the Monte Carlo method to get the uncertainty of diameter in circle fitted by the least square method. The method is more quickly as the less sample data involved.Comparison the experimental measure with the results by method proposed in the paper, it can draw a conclusion that the method can evaluate the diameter uncertainty accurately.