We carried out experiments on crystal growth from silicon melt with the FZ technique. We have precisely determined Si-30/Si-28 ratios of the obtained crystal using the IMS-1270 SIMS with Faraday cup multicollectors. Isotopic variations in the range of 1parts per thousand were observed along the growth direction of the crystal. We have observed the position of the interface between the grown crystal and melt during the crystal growth experiment using a CCD camera observation system. As we made a fast downward traveling of the crystal-melt system, the interface position and temperature fell quickly while the silicon isotopic ratio increased. It is inferred that the degree of supercooling of the silicon melt played a crucial role for controlling the isotopic ratio of the growing crystal. (C) 2004, Elsevier B.V. All rights reserved.