We have studied the propcrties of Ge0.96Si0.04 epilayers grown at high temperature on Si (0 0 1) substrates by X-ray diffraction, Raman spectra, and high-resolution transmission electron microscopy (TEM). Raman spectra show that there is an obvious transition region between the Ge0.96Si0.04 layer ang the Si substrate. The thickness of this region is about 100nm. Towards the Ge0.96Si0.04 layer, the Si fraction decreases rapidly from about 30 to 4%. In X-ray diffraction, there is a sharp peak of Ge0.96Si0.04 and a broad peak of the GeSi alloy region with lower Ge fraction. High resolution of cross-sectional TEM images show that the interface of GcSi/Si substrate is :1 1 Ij faceted.