3-dimensional phase-field model is developed to describe the cubic to tetragonal martensitic phase transformation in thin film attached to a substrate. Elastic solutions are derived for both elastically anisotropic and isotropic thin films with arbitrary domain structures, subject to the mixed stress-free and constraint boundary conditions. Nucleation process as well as final domain structure strongly depends on the substrate constraint. Lower undercooling results in less volume fraction of martensite, finer domain structure and longer nucleation period.