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This paper presents 10-Gbit/s binary phase-shift keying modulator and demodulator modules for a 120-GHz-band millimeter-wave wireless link. To reduce system complexity, we selected differentially coherent detection for the demodulator and designed the modulator and demodulator without IF circuits. The direct-conversion modulator consists of a 180° hybrid coupler, gain-control amplifiers, and a Wilkinson combiner. The demodulator has a 100-ps delay line made with coplanar waveguides and a variable phase shifter. The modulator and demodulator were fabricated with 0.1-μm-gate HEMTs lattice-matched to an InP substrate. A test element of the modulator exhibited a static vector error magnitude of 5%. We also mounted the modulator and demodulator in WR-8 waveguide modules and evaluated the characteristics of 10-Gbit/s data transmission. The bit-error rate for 10-Gbit/s pseudorandom binary sequence 27-1 data is 10-12 at a received power of -39 dBm.