The aim of this study is to develop a new measuring technique bsed on laser diffraction such as Fraunhofer diffraction theory which is called the diffraction gauge method. This method makes it possible to evaluate a three-dimensional cutting-edge profile of a micro-endmill with precision of better than 100nm even though on-machine measurement. The measurement resolution was experimentally achived to be 15nm for a helical endmill and repeatability of ±50nm with valid accuracy for new tool. In this paper, in order to verify that the diffraction gauge method is of practical use such as on-machine measurement, the three-dimensional cutting edge profile measurements of a worn tool with the diameter of 0.3mm, which has performed side milling on graphite with 40000 rpm rotational speed, is discussed. The experimantal results show that the diffraction gauge method has enough high performance to be able to evaluate wear quantity of a worn micro endmill.