This paper presents a high-resolution high-power free-space dielectric and magnetic measurement technique for evaluating the complex dielectric permittivity and complex magnetic permeability of solid materials in millimeter waves at room temperature. This method enables us to obtain broad-band high-resolution transmittance spectra using a quasi-optical free-space spectrometer equipped with an extended V-band (40-90 GHz) backward-wave oscillator as a high-power tunable source of coherent radiation. Due to the sufficiently strong energy throughput in the transmission and a very fine frequency sweep step of the measurement, this instrumentation is superior in obtaining reliable complex permittivity and complex permeability of absorbing dielectrics and ferrites with great stability and reproducibility over a much broader millimeter-wave frequency range. Simultaneous determination of dielectric permittivity and magnetic permeability can be achieved from a single set of measurements. Measurement results are reported for a variety of dielectrics and ferrites and compared with previously published data.