Measurement of complex permittivity of granular materials is important for varying services, such as remote sensing, telemetry, etc. In the literature, various techniques have been proposed to determine complex permittivity of granular materials sandwiched between two low‐loss spacers from transmission and reflection measurements. However, the determined results are non‐unique and resonant at some frequencies. In this article, a technique is proposed to determine unique and stable complex permittivity of granular materials. The application steps of the proposed are clearly presented. The grain and soil sandwiched between two polyvinyl chloride (PVC) spacers are measured in the X‐band to validate the proposed technique. In order to show the advantages of the proposed technique, a 140 mm X‐band waveguide blocked by two acrylonitrile butadiene styrene plastic (ABS) with thickness of 2 mm is measured.