The directional solidification of a (1 0 0) silicon wafer in <1 0 0> direction was carried out at various cooling rates. The planar to faceted melt/solid interfaces were observed, and the temperature profiles were measured by a mono-color pyrometer. The measured temperature profile for the planar interface is in good agreement with the analytical heat transfer model. Both undercoolings and thermal gradients in the melt and the crystal increased with the cooling rate, and the measured weighted thermal gradient G* was also consistent with the classical morphological instability theory. Moreover, in addition to the facet evolution with the increasing growth velocity, the nucleation and growth of the facet layers appeared from the facet tip and formed bunching steps near the facet groove.