Copyright © Proceedings of the 20th International Conference of the European Society for Precision Engineering and Nanotechnology, EUSPEN 2020. All rights reserved. An etalon is broadly used as temperature sensors, pressure sensors, and frequency filters. We developed all-fiber etalon and absolute long distance measurement method using etalon. However, it is confirmed that etalon optical length is a key component of the distance measurement method. In this report, we propose a new method of etalon optical length measurement using optical comb pulsed interference. In this method, comparison between interference fringes of pulsed interference and etalon reduces the uncertainty of the measurement. Results of experiments showed that 50 mm optical length was measured with several nano-meter repeatability with simple optical system. Furthermore, temperature effect on etalon optical length was observed. It was indicated that this method can be applied to optical fiber temperature sensor.