We observe an increase of air temperature in the world which, apart from negative effects, gives opportunities for growing new plant species. A good example of this phenomenon is soybean (Glycine max L.). Not only soybean reveals valuable nutritional properties but also the plant is an excellent element of a balanced crop rotation and is a good forecrop for follow-up plants, especially cereals. Soybean shows phytomelioration and phytosanitary properties. This plant has a well-developed root system which allows penetration of deeper soil layers. As a consequence, the soil is well aerated. In addition, soya cleans the field position off the stalk base diseases. The purpose of this thesis is to display the characteristic thermal conditions of soybean cultivation in Siedlce region from 1971 to 2015. The analysis of thermal conditions was based on daily, monthly and annual average air temperatures in Siedlce during the vegetation season (April-October). The sum of effective temperatures was also defined as the sum of daytime temperatures above 6, 10 and 15°C during the growing season. Basic distribution characteristics are specified in the article: arithmetic mean, minimum and maximum. The direction and significance of the trend of changes in analysed parameters were determined on the basis of linear trend equations. The significance of the directional coefficient of the trend was assessed with Student’s t-test at the significance level of α = 0.05.