In the experiment of Newton's ring， measuring the diameter of interference ring is the key and difficult step．It is confirmed in this paper that the type B uncertainty of the curvature radius of Newton's rings is affected when the parameter measured is the chord length rather than the diameter.The type B uncertainty can not be ignored when the linear correlation coefficient approaches to 1． In addition，the advantages are analyzed when measuring the chord length instead of the diameter on the basis of theoretical analysis，the method of calculating the type B uncertainty is also given．