Abstract: In order to investigate the failure mechanism of the microwave dielectric window applied in high-power tests and the influences of the deposited matter on its power-withstanding capability, the absorption curve of a thin contaminant film for the incident microwave power was calculated with the use of a simple physical model. The temperature rise on the dielectric surface caused by the contaminant was also estimated. It is shown that a contaminant film of several nanometers thick may absorb up to 50% of the incident power, even if the film thickness is only a small fraction of its resistive skin depth, which may be the possible major reason for the high-power microwave window failure.