The extremely precise level of cutting work's demand has been increasing, recently. Therefore, the establishment of a supreme measurement technology for micro endmills during operating on ultra-precision machining center plays an important role in many micro fabrication industries. The “Diffraction Gauge Method”, a novel measuring technique based on far-field laser diffraction, has been proposed to measure cutting-edge profiles of micro tools (φ<500μm) with repeatability of better than 100 nm even though on-machine surroundings. In this paper, cutting-edge profile measurement of worn micro endmills (φ300μm), which operated side-milling on graphite work material, is carried out. Agreement of the measured profiles with SEM image confirms the high accuracy of the measurement technique. Subsequently, the tool wear evaluations in various cutting lengths of micro endmills before and after cutting notify the relationship between increase in tool wears and cutting length. These experimental results verify the validity of our novel micro endmill measurement technique, which can be applied into ultra-precision machining center in the near future.