According to data of IPCC (Intergovernmental Panel for Climatic Change), the global surface air temperature increased to 0,6 ± 0,2 °C in the 20th century. Weber G. R., (1995) quotes that there is a trend of cold in the last 60 years in the middle latitudes including Europe, too. Starting from already mentioned perplexities we have tried to perceive the problem of climate variability in Serbia in the second half of the 20th century, when it came to very important increasing of concentration of CO2. With that aim we observed the decade values of average annual temperatures in the network of 20 climatic stations. In the period 1951 - 1990 a decrease of temperature was registered in 13 stations while in other stations an increase was less than 0,1 °C. Explorers from Bulgaria (Alexandrov V., 2000) and Hungary (Domonkos P., Zoboki J., 2000) came to similar results, too. However, if we take in account the last decade 20th century the number of stations with positive changes is enlarged on 15. Stations that have small changes and those with decrease of temperature were localized in the south and south eastern part of the country, and they are mainly coincided with before separated climatic regions with maritime pluviometric regime (Radovanović M., 2001). Using Dzerdzevskis B. L., (1975) division on three main types of circulation in the north hemisphere, we found that the increase of temperatures in the last decade 20th century is above all caused by change of dominant type of circulation from the south meridian to zonal. An analysis of seasonal changes showed that in the last five decades 20th century it came to decrease of winter temperatures in almost half of the stations in contrast with results of paleoclimatics models of possible greenhouse effect.