Ge1-xSnx alloys have been of growing interest, because it is a possible route to develop totally group IV based optoelectronic materials systems. Unstrained Ge1-xSnx alloy is predicted to exhibit a direct band gap, starting with x around 10%. Moreover, the presence of tensile strain is expected to decrease the Sn composition needed than unstrained alloys. Therefore, the ability to determine the strain ε and the Sn composition x, is of crucial importance. Raman spectroscopy is a very powerful technique for this purpose because the optical phonon modes of the alloys are strongly affected by x and ε. However, the strain and composition contributions are not decoupled. Other research groups published the composition dependence of Raman frequency of Ge1-xSnx alloys, but they corrected the strain contribution using coefficients for Ge. In our work, the strain of Ge1-xSnx alloys is controlled by growing on top of InGaAs buffer layers, whose lattice constants could be changed by In composition. XRD reciprocal space mapping of symmetric (004) and asymmetric (224) diffraction peaks of Ge1-xSnx alloys were carried out to determine the in plane and out of plane lattice constants. From the measured lattice constants, the Ge1-xSnx alloy composition and strain can be obtained applying a self-iterative simulation based on the elastic constants, which is assumed to change linearly with composition in the same manner as the lattice constant. From XRD data, we conclude that the strain of the Ge1-xSnx alloys ranges from -0.2% to 0.8%, and the Sn composition x ranges from 1% to 10%. Raman scattering experiments were performed at room temperature in the backscattering directions using 532 nm green Laser for the same set of samples. The Raman spectra of Ge1-xSnx alloys consist of one strong peak corresponding to Ge-Ge Longitudial Optical (LO) mode. For Ge1-xSnx alloys of the same Sn concentration, this peak progressively shifts toward lower energy and broadens with increasing strain. The frequency shift with respect to the value for bulk Ge, ωGe=300cm-1, follows the linear relationship ∆ω=ω- ωGe=bε, where ε is the in plane strain. By comparing the Raman shift and the strain data from XRD, the coefficient b is determined as - (570±7)cm-1. This coefficient is purely for strain contribution. After deducting the strain contribution from total Raman peak shift using this coefficient, the coefficient of the composition dependence of Raman frequency of Ge1-xSnx alloys is calculated. The value is - (80±5) cm-1.This value is closed to the value from literature, - (75.4±4.5)cm-1.