This paper presents a simple and low cost method to estimate relative permittivity and tangent loss for dielectric sheets used in printed circuit boards (PCB's). The process starts by building a rectangular cavity in a very easy manner. As an example, the results to estimate relative permittivity and tangent loss of a fiber glass PCB are shown. The relative permittivity and tangent loss are presented as a function of frequency. To obtain resonant frequencies, the return loss (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> ) was measured using a vector network analyzer (VNA). Relative permittivity and tangent loss were estimated at the frequencies of the excited modes in the resonant cavity. The results are presented in a frequency span of 1 GHz to 11 GHz. The nonlinear behavior of the relative permittivity and tangent loss for the dielectric material evaluated are presented. The relative permittivity exhibites a stable behavior at high frequencies while the tangent loss is shown to increase with frequency.