Three layers of fully-strained Ge1-xSnx alloys with x=0.025, 0.052, and 0.078 from bottom to up are grown on a Si (001) substrate using a high-quality, strain-relaxed Ge thin film as buffer layer. Five relaxed Ge1-xSnx samples (x=0.005, 0.016, 0.044, 0.070, and 0.155) are grown directly on Si (001) substrates as well. The compositions and lattice constants of the Ge1-xSnx alloys are measured by Rutherford backscattering spectra, high-resolution X-ray diffractions, and X-ray reciprocal space mapping. The experimental results reveal a quite large positive deviation from Vegard's law with a bowing parameter b=0.211 Å.