We observed the variations of the semi-annual oscillation (SAO) in the mesopause region of northern subtropics, and we describe its origin and forcing implications. Using the measurements of horizontal wind profiles made by the University of Illinois meteor radar in Maui, Hawaii (20.7°N, 156.3°W) from May 2002 to June 2007 and the European Centre for Medium-Range Weather Forecasts (ECMWF) interim data set, we find that the mesospheric SAO, with the winter westerly near 80–90 km clearly stronger than the summer westerly, is out of phase with the stratospheric SAO near 1 hPa (∼50 km). The mesospheric SAO easterly is strong during the easterly phase and weak during the westerly phase of the stratospheric quasi-biennial oscillation (QBO) near 10 hPa (∼30 km), suggesting the modulation of the mesospheric SAO by the stratospheric QBO. The mesospheric QBO has an amplitude of approximately 5 m/s near 80 km. It is in phase with the stratospheric QBO near 10 hPa and out of phase with the QBO-like oscillation near 1 hPa. The correlation of the gravity wave (GW) and the quasi-two-day wave (QTDW) activities with the mesospheric SAO and QBO suggests that the GW drags and the QTDW Eliassen-Palm flux divergences likely contribute to the QBO modulation of the mesospheric SAO. The winter easterly wind in the tropical upper stratosphere pushes further into the northern subtropics during the QBO westerly phase than during the easterly phase. This may have impacts on the upward propagation of westward-propagating GWs originated from the middle latitudes, and thus the westward GW forcing in the upper mesosphere of northern subtropics.