This paper presents the classification of thermal periods based on the dates of passing through the conventional thermal thresholds. The dates of the beginning and the ending were determined along with the number of days of the following thermal periods: agricultural, vegetation and active plant growth period. The analysis was based on data concerning mean daily air temperature values coming from nine IMGW stations located in the region of central-eastern Poland, covering the period of 1971-2005. It was noticed that in the examined area there occurs spatial differentiation of the number of days as well as of the beginning and ending dates of particular thermal periods. Agricultural period in central-eastern Poland began on average on March 13^th and lasted till November 12^th. This season was the shortest in the eastern part of the area examined, while it was the longest in the western and southern part. Thermal vegetation period was characterised by a tendency to begin earlier and end later. The average duration of vegetation period in central-eastern Poland lasted form 210 to 222 days. In northern part of the area examined this period was the shortest, while it was the longest in the western and southern part. The analysis of directional coefficients of the linear trend in duration of the active plant growth period revealed that dates of its beginning during the examined years did not show any significant variation. Whereas, its ending dates varied significantly at all the stations. This season was the shortest in the north-eastern part of the area examined, while it was the longest in the western and southern part.