A device and method for the non-invasive measurement of blood glucose concentrations by millimeter waves. The device includes a millimeter wave generator; a TE10 mode rectangular waveguide transmitting a millimeter wave generated by the millimeter wave generator; and a plane parallel plate chosen to yield a minimal point of the power reflection coefficient of the millimeter wave incident to and reflected from the dielectric object under test via the TE10 mode rectangular waveguide and the plane parallel plate. The device also includes power detectors detecting the powers of the incident wave generated by the millimeter wave generator and the reflected wave from the dielectric object via the plane parallel plate; a temperature sensor measuring a temperature of the dielectric object; and a reader reading the minimum power reflection coefficient and a corresponding frequency from the incident and reflected wave detected by the power detectors.