To mimic a conventional optical reflection and transmission detection method to measure the dielectric constants of bulk materials, we develop a microwave double dielectric resonator to measure the dielectric constants of nanometallic powders. The vacuum evaporated metallic nanoparticles mixed with a low loss tangent alumina powder, exploited to reduce the rapid attenuation of electromagnetic field, are used to fill the inner hole of a sapphire tube by which the resonant frequency and Q factor are measured at the TE011 mode to derive the dielectric constant. The experimental results for large grain size particles are comparable to the bulk optical values.