A new optical measuring method of cutting edge profiles with a few nanometerresolutions for micro milling tool is proposed. The edge profile is scaled by the width between the first order maxima in a diffraction pattern, which relates to the spacing of a slit-type aperture formed between a reference knife-edge and a tool cutting-edge. The technique is applicable for on-machine measurement due to simplicity and long working distance of the optical system. In this paper, first, the theoretical analysis reveals the measurement resolution, and then the proposal of laser diffraction gauge method is mentioned. Next, the calibratability and the practicability of our technique are affirmed with fundamental experiments. Furthermore, experimental verifications are carried out for commercial tools such as straight-flute and helical flute of micro endmills. Subsequently, excellent agreement of the profiles, which are measured with our developed experimental systems, with an image from scanning ion microscope verifies that the proposed method is efficient to accurately measure cutting-edge profiles.