The materials used in microwave oven cavities must have specific dielectric properties in order to maintain the efficiency of the food heating. Plastics, by their mechanical and chemical properties and low cost, are one of those potential materials. In this study, we present the results of the measurements of complex dielectric constant, epsilon(*) =epsilon'-i epsilon '', in the microwave frequency region, on different plastics: polyoxymethylene (POM), polypropylene (PP) and polybutylene terephtalate (PBT), using the cavity resonant method. We measure the shift in the resonant frequency of the cavity, Delta f, caused by the insertion of the sample, which can be related to the real part of the complex permitivitty, epsilon', while the change in the inverse of the quality factor of the cavity, Delta(1/Q), gives the imaginary part, epsilon ''. The relations are simple when we consider only the first order perturbation in the electric field caused by the sample.