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High-resolution frequency-domain spectroscopy (FDS) is set up using a coherent and continuous wave terahertz (THz) emitter and receiver. THz waves are generated and detected by two photomixers with two distributed feedback (DFB) lasers. Atmospheric water vapor with different relative humidity is systematically investigated by the FDS. A high-frequency resolution of ～14 MHz is obtained with the help of Hilbert transformation, leading to a well resolved and distinct transmittance characterization of water vapor. Compared with conventional THz time-domain spectroscopy, the high-resolution continuous wave THz spectrometer is one of the most practical systems in gas-phase molecular sensing, identification, and monitoring.