The semiannual oscillation (SAO) of the terrestrial thermosphere, which has the equinoctial maxima both in the polar and equatorial regions, shows a striking hemispheric asymmetry. The maximum amplitudes of the thermal SAO at 300 km are 26.7¿K and 15.8¿K for the northern and southern polar regions, respectively. The corresponding SAO amplitudes of the neutral N2 density are 10.5% and 3.7%, respectively. Furthermore, the phase of the southern hemispheric SAO lags behind the phase of the northern SAO; i.e., the maximum of SAO at 90¿S is delayed from that at 90¿N by approximately 55 days in the temperature and 25 days in the N2 density. These values are obtained from mass spectrometer (17 satellites) and incoherent scattering observations (4-radar stations), and from several rocket-soundings (Heding, 1983). The larger thermospheric SAO amplitude in the northern hemisphere over that in the southern hemisphere, as well as the phase-differene between the two hemispheres can be ascribed to dynamical heating by upward propagating small amplitude internal gravity waves (IGW), in addition to the well-established sources for the thermospheric SAO, i.e., solar EUV heating at low latitudes and auroral heating at high latitudes. The upward energy flux transferred by propagating IGW increases during the transition periods between summer easterlies and winter westerlies, which occur twice a year around the equinoxes. Due to the orography, the atmospheric wave activities are stronger and the zonal winds are weaker in the northern hemisphere than in the southern hemisphere. Correspondingly, the contribution from dynamical heating by IGW to the thermospheric SAO is larger in the northern hemisphere than in the southern counterpart, and the zonal wind reversal is delayed in the southern hemisphere relative to the northern hemisphere. We propose that this may explain the larger amplitude and the phase-advance of the thermospheric SAO in the northern hemisphere relative to the southern hemisphere.