We discuss the structural changes in Si from an undercooled state to a crystalline state during crystallization from the analysis of structural information obtained using high-energy X-ray diffraction experiments combined with the electromagnetic levitation technique. From the structure analysis of undercooled molten silicon, we found that the short-range order of the undercooled molten silicon was not drastically changed from that of the high-temperature state above the melting temperature, while the medium and/or long-range order over the third or fourth neighbor would be changed with undercooling. Moreover, from in situ observations of the crystallization of Si from the undercooled state, we found that the atomic structural arrangement formed a diamond structure even at the initial stages of crystallization from the undercooled state. Based on the experimental results, we propose distortion of the atomic structural arrangement of Si from the modified beta-tin structure to the diamond structure during nucleation in the undercooled state. (c) 2006 Elsevier B.V. All rights reserved.