Forest vegetation of Serbia is adapted to specific climatic conditions prevailing in the area. These conditions change over time as reflected in the growth, development and vulnerability of all organisms. Value and variability of climate elements indicate when and to what extent there is a danger of the spread of fire in the woods. The possibility of prediction of forest fires, among other things, based on climate data and monitoring the application of various models. One of these models is Lang’s rain factor (Kf), which was used in this paper. Analysis of this factor was performed in 26 cities in Serbia. The aim of this study was to investigate the monthly and annual fluctuations in rain factor during the period 1981-2010 compared to the standard climatological period (1961-1990) and to identify areas and periods of increased forest fire danger. Values of Lang’s rain factor decrease in the area of research, which indicates an increase of aridity in the region of Serbia and the increase of threat of forest fires.