In situ observations on the melting processes of non-doped silicon were carried out by X-ray topography method. The dislocation-free melting process was successfully obtained through the appropriate accommodation of the shapes of sample and heaters. To clarify the dislocation effect on the melting process, the melting process with an isolated dislocation in the melting zone was studied. It is shown that the melting processes with and without an isolated dislocation are different for the non-doped silicon crystal. The crystal-melt interface shape is estimated from the digitized contrast of the topography image. The crystal-melt interface shape is kept flat in the dislocation-free melting process, while the interface shape is rough in the melting process with an isolated dislocation in the molten zone. (C) 2002 Elsevier Science B.V. All rights reserved.