A method of measuring the relative complex permittivity ( ɛ r = ɛ′ – jɛ″, tan δ = ɛ″/ ɛ′) for low-loss dielectric materials at millimeter wave frequencies has been developed, using a dielectric rod resonator excited by the nonradiative dielectric waveguide (NRD-guide). Relative permittivity ( ɛ′) and loss factor (tan δ) of the rod specimen are determined by the resonant frequency ( f 0) and unloaded Q-factor ( Q u) of a TE 0m1 mode resonator. The effective conductivity ( σ) of conducting plates for short-circuiting the rod resonator is determined using TE 021 and TE 02 δ mode sapphire resonators. Temperature dependence of ɛ′ and tan δ of sapphire and cordierite ceramics were evaluated at 60 GHz. This method has been adopted as the Japanese Industrial Standard (JIS R 1660-3) and is being prepared for the IEC international standard. Several standardized specifications are presented.