We present theoretical study on spin reorientation transition (SRT). A phenomenological model is provided,
which treats magnetic energy as sum of magnetoelasticity, magnetocrystalline anisotropy, and demagnetization
contributions. We show that critical strain exists by analytically solving the phenomenological model, as a consequence
of an interplay between aforementioned contributions. For more quantitative understanding, numerical
estimate is performed for thin FePt and Fe60Co40 films, where experimentally accessible critical strain is 2
%. Further, this feasible strain is due to the large magnetoelastic coefficient (b1) and the optimal saturation
magnetization, which is not achievable in thin Fe film.