The imaginary part of the dielectric constant ε" and of the permeability μ" of the sample were determined by measuring the change of reflection coefficient of a resonance cavity of the reflection type with and without the sample in it. The effects of the shape of the sample and its location in the cavity on the measurement were investigated. The sensitivity of the measuring system was high. By raising the quality factor of the cavity, adjusting it to approach critical coupling, and appropriately choosing a cavity of smaller volume, it should not be difficult to measure the loss factor with this set of measuring devices to an accuracy of the order of 10-5.