The ferroelectric-paraelectric phase transition was investigated in BaTiO3 100nm thick film deposited on platinized Si substrate by measuring the lattice parameters as a function of temperature and the hysteresis loop at room temperature. The transition from paraelectric to ferroelectric phase takes place at ∼420K (instead of 400K for unconstrained single crystals), whereas the film is under high tensile elastic in-plane strain (0.34%) and high inhomogeneous strain (0.69%) along the growth direction. Comparison of the experimental results with recent theoretical calculations suggests a monoclinic ferroelectric phase at room temperature, where interface “dead layer” plays a key role.