We have studied the mechanism of strain relaxation of Ge0.3Si0.7 alloy layers grown on low-temperature (LT) Si buffers by high-resolution X-ray diffraction and transmission electron microscopy (TEM). X-ray rocking-curve analyses show that the strain relaxation of an alloy layer is quite inhomogeneous at its early stage, and becomes homogeneous when the layer approaches becomes fully relaxed. High-resolution cross-section TEM analyses indicate that stacking faults hale formed in the LT buffer near the interface of LT-Si/GeSi and have separated the mismatch dislocations in the interface of GeSi/LT-Si into Shockley partials. We propose that this could be due to the aggregation of the vacant defects ill the LT-SI layer. The mechanism of this aggregation is the osmotic force created by the vacancy super-saturation in the LT-SI layer and/or the tension stress propagating from the interface of GeSi/LT-Si.