The optical transition characteristics of fully strained Ge_1−xSn_x films grown on Ge substrate were investigated by high-resolution X-ray diffraction (HRXRD) and spectroscopic ellipsometry (SE). The results showed Sn composition-dependent nonlinear behaviors in interband transition energies. The influence of strain on nonlinear behaviors was identified by the ratio of bowing parameters. Optical transition energies are largely tuned due to the strain-induced band structure. The strain in GeSn alloys may be responsible for the fluctuation of interband transition energies. The effect of full strain appears to result in an opposite trend in the direct and indirect band gap energies. The transition from indirect-to-direct band gap semiconductor in the present work is determined to be x = 0.103 at 300 K. These results contribute to further exploration into band gap engineering for mid-infrared optoelectronic materials.