The nose roundness is one of the most important indices of lapping quality of circular-arc diamond cutting tools. Because of its special requirements, until now there was no efficient method to measure the nose roundness. In this article, a new way, based on atomic force microscope and precision-revolution axis, to measure the nose arc of tools is proposed. A radius constraint least-squares circle fitting method is put forward. The metrical data are fitted by the nonlinear fitting function Lsqnonlin in software matrix laboratory (MATLAB). Also, the peak-valley value is used to assess the nose roundness. The method fully meets the needs of measurement and evaluation of circular-arc diamond cutting tools' nose roundness.