Measurements and calculations of longwave radiation fluxes were made in order to clarify their characteristic features under a surface inversion and with katbatic winds. Measuements were made under the Japanese POLEX program (POLEX-South), 1979-1981, at Mizuho Station (70¿42'S, 44¿20'E), where the katabatic wind was blowing continuously and a strong surface inversion existed. Direct measurements of the downward and upward longwave fluxes were extended through day and night using pyrgeometers (Eppley PIR) with a simple shading ring to cut off heating by direct solar radiation. Longwave radiation fluxes were also calculated using a simple wide band model, and the effect of the surface inversion was examined. The daily average of the downward flux varies greatly between 90 and 240 W/m2. The large day to day variation is due to clouds. Overcast skies give an increase of about 80 W/m2 in the downward longwave flux in all seasons. The effective emissivity of the atmosphere for clear sky is very small on account of the small amount of column water vapor. The drifting snow near the surface caused by the strong wind increases the downward longwave flux and suppresses the longwave cooling at the surface. The relation between the strength of the surface inversion and longwave fluxes is examined and approximated by a linear relation. When the surface inversion becomes strong, the net longwave flux decreases.