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Ba0.5Sr0.5TiO3 (BST) thick films for RF MEMS applications were prepared by a modified sol-gel method and characterized by microwave measurements. In order to obtain thick films and low loss tangent, poly vinyl-pyrrolidone (PVP) and Mg were doped into the solution to avoid crack formations and reduce loss tangent. Interdigital capacitors and CPW microstrip lines for low and high frequency measurements, respectively, were patterned on BST films. Their theoretical models with three-layer structures were established with conformal mapping techniques to extract dielectric properties of BST at frequencies from 50 M to 26 GHz. Experiments show that PVP and Mg are effective in improving the performance of thick films. Applications of BST thick films to RF MEMS devices were discussed.