In this paper, a dielectric properties extraction method for millimeter-wave applications is presented. Substrate integrated waveguide (SIW) cavity resonators with the same structure and varied thicknesses are employed to separate the dissipation factor (DF) of the substrate material for the cavity resonators. The dielectric constant and loss tangent of the dielectric substrate for the SIW is extracted at the resonance frequencies based on the unloaded Q-factors of transmission loss measurement. The DF from the unloaded Q-factors, which is highly dependent on the thickness of the substrate, is extracted using an iterative fitting process for the substrate thickness estimation without cross sectioning. To validate the extraction method, the SIW cavity resonators are fabricated using RO4003C substrate material and the dielectric properties are extracted in the X-band (8.2 to 12.4 GHz). The extracted thicknesses of the SIW resonators are validated by cross-sectioning. Additionally, the extracted dielectric properties are also verified by comparing the dielectric characteristics of the SIW resonators with the different thicknesses. With the presented method, the time expense for the conventional dielectric characterization method with cross-sectioning is reduced.