Satellite images have suggested the presence of winter open water, or a polynya, off the Riiser-Larsen Peninsula (around 68.5°S, 35.0°E) in East Dronning Maud Land, Antarctica. To ascertain evidence for polynya formation and to survey thermal structures, the 41st Japanese Antarctic Research Expedition executed aircraft observations in 2000-2001. The observations have verified that the polynya appeared and sea ice formed in the cold season. In the polynya region, water temperature profiles were measured using an Airborne-XBT device. It is found that the winter convective-mixed layer which cooled to its freezing point reached the depth of about 250 to 300 m. The mixed layer was thinner than that of another polynya located over the shelf break around 68.5°S, 39.0°E.