Abstract: For the determination of dielectric properties of low-loss materials in millimeter wave band, this paper describes a biconcave quasi-optical resonator with the Q factor over 0.8 million, which enables the determination of loss tangents down to the 10-5 range. The biconcave resonator system utilizes a phase-locked loop with harmonic mixer to synchronize a backward wave oscillator source and a microwave spectrum analyzer. The spectrum analyzer acquires high-resolution scan of the resonance curve. The Q factor of the cavity is calculated by performing a non-linear least squares fit to a Lorentzian line shape or by the resonance curve area method. The test results of quartz, CVD diamond and sapphire at ambient temperature are close to the data in the literatures, and the permittivity of 4H-SiC at 132.07 GHzare measuredto be 9.598 and the loss tangent to be 6.110-5。