Gauge block measurement by using optical interferometry in Korea Research Institute of Standards and Science (KRISS) is described. A partially modified commercial Twyman-Green type gauge block interferometer equipped with three frequency stabilized lasers which are coupled into one single mode optical fiber, is used for the measurement of gauge blocks of nominal length up to 250 mm. Fringe scanning Fourier transform method is used to obtain the excess fraction value from the interference fringes. The temperature inside the interferometer is stabilized within -4 mK for three hours. The standard uncertainty (k=1) of measurement is 29 nm for a 250-mm gauge block.