The grain boundary YBa2Cu3Oy(YBCO) Josephson junctions are fabricated on (100)MgO bicrystal substrates with a misorientation angle of 24° and the microwave properties of Josephson effects are studied up to millimeter‐wave range. Strong [100]//[010] and [010]//[100] in‐plane orientations are observed at the junction interface of a YBCO film, whose microscopic geometries are discussed in connection with the observed Josephson effects. The directly observed maximum Josephson microwave self‐radiation powers from the junction at receiver frequencies offREC=22 GHz and 47 GHz are 3.2×10−12W and 5.6×10−14W, respectively. The observed characteristic voltage (Vc) is 4.4 mV, corresponding to the frequency of about 2 THz at 4.2 K. ©1995 American Institute of Physics.