A method to determine simultaneously the magnetostrictive coefficient and the elastic properties (Young's modulus and Poisson ratio) of a thin film during the growth was developed based on minimization of the total elastic energy of a cantilever film-substrate system. Compared to other published measurement methods, an inaccuracy in the magneto strictive coefficient, caused by assuming the elastic properties of the film as those of the bulk material, could be avoided. Only one elastic isotropic substrate is employed in the present method. The experimental data of an Fe-based amorphous thin film was analyzed using the model. The calculated dependence of the magneto strictive coefficient on the external magnetic field was compared with the experiment, and the discrepancy between both results is explained. Furthermore, the elastic properties of the film were also obtained.