Polycrystalline (Ba 0.70 Sr 0.30)TiO 3 (BST) thin films were prepared by ion beam sputtering and postdeposition annealing method, and their crystallization kinetics was monitored in situ. It was found that 550 1C is a critical temperature for the BST's perovskite phase formation, below which a long annealing time is needed to obtain good perovskite structure because of a slow crystallization process, while generating much lower inhomogeneous and thermoelastic strains. As a consequence, the ferroelectric/paraelectric state formation in BST films, being strain sensitive, was purposefully tailored through simple kinetics instead of pure thermodynamics. A long time annealing at 530 1C induced a room temperature (RT) ferroelectric state, whereas the annealing at 550 1C and above promoted much faster perovskite crystallization and tailored the film at RT in its paraelectric state, thereby giving rise to two distinct dielectric behaviors. These findings provide important implications for simply controllable state growth of perovskite thin films with desired electrical properties so as to meet their respective requirements in different application fields. &