This work describes the use of frequency-modulated continuous-wave (FMCW) radar sensors for millimetre wave measurements to extract the dielectric constants of different types of dielectric materials. The FMCW radar measurements were done in the frequency range 122 GHz 169 GHz, and offer a fast and accurate measurement at much lower cost, especially at this high frequency range, when compared to a standard vector network analyzer. The extraction of the permittivity of dielectric materials is based on a monostatic radar measurement of the reflection coefficient by using a free space setup. Therefore, a special measurement set up to operate at oblique incident angles was presented. The measurements setup will be described, as well as its implementation and calibration. To validate the reflection coefficient measurement results, an analytical model was implemented and presented. Finally, we show that our measurement setup system allows to characterize dielectric materials with high precision. The results from material characterization measurements in the millimetere-wave (mm-wave) was done for the following materials: Polytetrafluoroethylene (PTFE), polyvinylchloride (PVC), nylon, and gypsum.