(Ba,Sr)TiO3 (BST) thin films have been investigated because of significant interest in device applications of their nonlinear dielectric properties such as in frequency tunable devices. We have investigated frequency conversion devices using BST films as new nonlinear devices, such as frequency mixers and multipliers. In this paper, we report the evaluation of nonlinear properties of various capacitors, and the experimental equation of the properties and the equivalent circuit model for frequency converters were established. A frequency multiplier using the BST film capacitor was fabricated, and obvious third harmonic signals could be observed. A conversion efficiency of -20 dB was obtained with a low input power of 0.25 W. Analysis of the converter with an equivalent circuit was also carried out, and the calculated output voltage agreed well with the measured result. The model is expected to be used for the analysis and improvement of conversion devices.