We report on the measurement of the millimeter wave transmission properties of a dielectric waveguide with a YBa2Cu3O6+xboundary layer. The transmitted power and phase shift of a 94 GHz signal propagating through the dielectric waveguide structure was measured over the temperature range of 77–150 K. The transmitted power was observed to depend upon temperature due to the dissipation of the millimeter wave signal in the YBa2Cu3O6+xboundary layer. Thereby the temperature dependence of the normal‐state ac conductivity was measured. The experimental data were fitted to a Drude model for the normal‐state ac conductivity of the YBa2Cu3O6+x. The temperature dependence of the Drude scattering time was obtained. The scattering time was found to decrease with decreasing temperature and was 7.5–11.2 ps in magnitude.