In situ observations of the melting processes of dislocation-free silicon crystals were carried out using x-ray topography technique. Heating procedures with various heating rates were used in the experiments. Dot contrasts were observed during the melting process at a high heating rate, while they could not be found at a low heating rate. It was found that the melting process at a high heating rate is spatially inhomogeneous, while that at a low heating rate is homogeneous. It was also found that the oxygen concentration in the sample plays an important role in the formation of dot contrasts during the melting process. The dot contrasts were assumed to originate from the dislocation loops around the oxygen precipitates.