In this paper, we reported an ultra-sensitive millimeter wave reflection liquid sensor, which is electrically tunable, non-invasive and label-free. The reflection liquid sensor is based on the impedance match technique and implemented in a rectangular waveguide structure, which makes the interaction between millimeter wave and liquid under test (LUT) in a closed environment, forms a nano-fluidic lab-in-waveguide liquid sensor, and isolates the interference of surroundings. The reflection liquid sensor shows high sensitivity to the small complex-valued permittivity difference of the LUTs. Taking the ethanol-distilled water as the benchmark, a 64-dB ethanol-distilled water contrast is experimentally achieved. The detection limit is 0.05% ethanol in volumetric concentration with a 4-dB signal difference, which is the best performance of the microwave and millimeter wave liquid sensors publicly reported until now. The ultra-sensitive reflection sensor can be used for bio-chemical sensing. The lab-in-waveguide biosensor is also compatible with microfluidics.