We recently achieved heteroepitaxial growth of high-quality 3C-SiC on a 6H-SiC 0001 plane using an Si-Sc solvent. The present study seeks to determine the polytype transition mechanism from 6H to 3C that occurs during growth. 3C-SiC grows by 2D nucleation, which gives rise to flat domains, whereas 6H-SiC grows by spiral growth originating from threading screw dislocations in the seed crystal. 3C-SiC expands laterally, covering the spiral growth of 6H-SiC. This preferential growth of 3C-SiC can be explained in terms of geometrical selection due to different growth rates of the two polytypes. We developed a simple model that considers the step height and the step density. It predicts that growth of 3C-SiC by 2D nucleation will have a higher growth rate than 6H-SiC spiral growth in a certain supersaturation range. (C) 2011 Elsevier B.V. All rights reserved.