The importance of the beta-FeSi2 bulk single crystals has increased not only to investigate the intrinsic properties of beta-FeSi2 but also to use it as substrate of beta-FeSi2 thin films for optical devices. Though single crystals of beta-FeSi2 are grown by chemical vapor transport (CVT) method, most of those crystals are needle-like and widths of those flat surfaces are 0.5 nun or less. In order to understand the mechanism of the growth process of beta-FeSi2 by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the experimental data, we have proposed the most likely reaction process, FeI2(g)+2Sil(4) (g)-> FeSi2(s)+ 5I(2)(g), and we found that the crystal growth progresses under the environment where the FeI2 gas is insufficient compared with a suitable ratio of FeI2/SiI4. Then, to raise the partial pressure of FeI2 gas, the composition ratio of Fe to Si for the source material was increased and we have obtained the plate-type beta-FeSi2 crystals that exceeded a few square millimeters in size. (c) 2007 Elsevier B.V. All rights reserved.