Abstract: Phase stability and frequency-dependent ferroelectric and dielectric properties of epitaxial Ba_0.6Sr_0.4TiO₃ thin films under different misfit strains were investigated using a modified Landau-Devonshire phenomenological theory combined with Landau-Khalatnikov equation. The phase diagram analysis indicates that ferroelectric tetragonal (c), orthogonal (aa) and paraelectric phases are stable at room temperature and all phase transitions are of second order. The compressive misfit strain increases out-of-plane remnant polarization (P_r) and coercive field (E_c) in c phase while the tensile misfit strain augments in-plane P_r and E_c in aa phase at an electric field frequency of 0.1 kHz. Moreover, it is found that both polarization and coercive field increase with increasing frequency while the dielectric constant decreases rapidly first and then slowly. The maximum tunability decreases and the corresponding misfit strain gradually approaches zero, which is attributed to the enhancement of phase transition temperature (ΔT_c). The further calculation results reveal that the shift of misfit strain, decrease in tunability and ΔT_c are 0.017%, 10.6% and 7.3 ℃ respectively in the out-of-plane case, which are comparable to 0.019%, 9.7% and 7.7 ℃ respectively for the in-plane case.