The flow instability of molten silicon in the Czochralski configuration has been studied by in-situ observation of melt convection using X-ray radiography and by temperature fluctuation measurement during crystal growth. Flow mode was dependent on an aspect ratio of the melt. For a deep, low aspect ratio melt, with growing crystal which is identical to shouldering process of the growth, the flow was unsteady and non-axisymmetric. For a shallow melt without crystal and crucible rotations, the flow was relatively steady and axisymmetric. However, flow became unsteady and non-axisymmetric for a shallow melt with crystal rotation. Amplitude of directly measured temperature fluctuation in the molten silicon for the case of unsteady and non-axisymmetric flow was larger than that for the relatively steady and axisymmetric flow. The flow instability area, which was also thermally unstable, was found to be larger in the crystal/crucible iso-rotation condition. In contrast Munakata and Tanasawa reported at the International Symposium on Supercomputers for Mechanical Engineering, September 1988, Tokyo that flow instability area was small for the silicone oil with larger Prandtl number.