A novel technique is presented for the measurement of the absorption properties of liquids in the terahertz (THz) wave range, based on the interaction between the liquid and the evanescent wave covering the surface of a silicon waveguide. The waveguide is a high-resistivity silicon cylindrical rod (resistivity above 1000Ω・cm, 50mm in length, 2mm in diameter). The THz wave propagates inside the silicon waveguide by total internal reflection as the fundamental EH_<11> mode and generates an evanescent field covering the silicon rod. The presence of a medium around the rod has a measurable effect on the overall end-to-end transmission of the waveguide. As an application, we report on the monitoring of a D-glucose aqueous solution with a concentration ranging from 0% to 45%, using a backward-wave oscillator as the THz wave source. As another application, the real-time label-free sensing of avidin-biotin binding was also observed. According to the changing of the transmittance of the THz wave, the process of the avidin-biotin interaction can be observed on time. The advantage of this technique is that it can extract some information from the highly absorbing liquid sample in the THz range without transmitting sample itself. While still at the experimental stage, it is believed to be a rapid technique for the measurement of other liquid sample in THz range.