Radon concentration and cave temperature were studied at two morphologically different locations (the Beautiful Caves—BC and the lowest point—LP) in Postojna Cave (Slovenia). Radon concentration was considered a trace gas for cave ventilation research, where cave temperature was used as a supported microclimatic parameter. The influence of several climatic factors on radon concentration was studied, from which outside air temperature was found to have the highest control over cave ventilation. Measured radon concentration during 1-year period was in the range from 740 to 4000 Bq m−3 at LP, whereas during 2-year period at BC radon concentration experienced two different annual cycles. The change was observed after flood in September 2010. Radon concentration was in the range from 500 to 9000 Bq m−3 in the year 2010 and 500 to 5500 Bq m−3 in the year 2011 and 2012. Temperature was in the range from 9.9 to 10.5 °C and 9.8 to 10.7 °C at BC and BC-1 (the main passage connected to the LP), respectively. Two different ventilation regimes can be distinguished for LP location and for year 2010 at BC, whereas after flood, three ventilation regimes are characteristic for BC location. Significant change of radon emissions was observed after flood.