The influence of loaded capacitance on the resonant frequency of a triplate-type strip resonator is investigated through numerical simulations by means of the finite-difference time-domain (FDTD) method. This type of resonator is one of the basic components of very small high-dielectric stripline filters, named laminated planar filters. Numerical results of resonant frequencies are compared with experimental results and found to be in excellent agreement, which circumstance ensures that the FDTD method can be applied to the characterization of a wide range of laminated planar microwave devices including resonators and filters. It is also found that the resonant frequency is directly related to the square root of its line capacitance when the resonator is regarded equivalently as a series LC circuit.