This paper proposes to use microwave heating in molding processes of resin powder for improving the heat efficiency and reducing the processing time. Simple measurement methods of distributions of electric field intensity in various types of cavities and loss coefficients of resin powder with multicomponents were developed. The electric field intensity can be determined from the rising rate of temperature of water whose loss coefficient is known. This measurement showed a large distribution of electric field intensity in a cavity, but the distribution was improved by the stirring of a fan. The microwave absorption performance of the powder resin of a PVC base is superior to the pure PVC since it is much influenced by the plasticizer contained in the sample than PVC itself. The preheating of the sample enhances the energy absorption and it may allow the processing time to be reduced considerably. The loss coefficients of the samples are measured by the method proposed in this work. The effect of additives on the loss coefficient of the samples consisting of multicomponents is studied as well, and it is implied that the apparent coefficient is not successfully correlated in simple proportion to the weight and the loss coefficient of each component.