Using high-TcJosephson junctions made ofYBa2Cu3O7−&dgr;deposited across MgO bicrystal boundary, at terahertz (THz) frequency band, we demonstrated both fundamental and harmonic mixing. Radiation from a far-infrared laser was coupled to the junction, which was integrated with a planar bow-tie antenna, via an extended hyperhemispherical silicon lens. Fundamental mixing manifested itself in the junction’s dc current–voltage (I–V) curve as a third Shapiro step in addition to those two induced by the THz laser lines from a slightly misaligned resonator. In harmonic mixing between a THz laser line and a microwave local oscillator, the highest harmonic number we could get was 490 with a signal-to-noise ratio of 9 dB at the intermediate frequency. ©1997 American Institute of Physics.