The bias-field-dependent dielectricity of ferroelectric thin films is studied by use of a multilayer model capable of modeling saturated and unsaturated hysteresis behavior under arbitrary fields. The simulated minor hysteresis loops at different bias fields are used to calculate the variation of dielectric permittivity with the bias field. The ∊-E loops show asymmetric shifting along the field axis when the film is assumed to possess a secondary dielectric layer with trapped charge. Simulated D-E and V-E loops are compared with the experimental data on barium zirconate titanate and lead zirconate titanate thin films. In general, the model predictions show reasonably good agreement with experiment. Effects of charge density and the ac measurement field amplitude on the measured permittivity, as well as the dielectric tunability have also been examined.