In this study to determine the natural seasons of the Southern Caspian Sea, data average for the maximum and minimum temperature and average monthly area for 13 stations in the range were studied for a period of 20 years (1986 to 2006). To analyze the data the harmonic and cluster analysis methods were used. The results of the data analysis show that in the south of the Caspian Sea the year can be totally separated in three main seasons: hot, cold and transitive. In most stations, each of the hot, cold and the total cold and transitive seasons is composed of four months, but the natural seasons in the south of the Caspian Sea do not match with the beginning of the seasons’ calendar. The number of the months related to the hot seasons for every station is about four or five months and is included the months June, July, August and September and the months related to the transitive season are included in some stations between two to five months and they are April, May, October, November and December. In the end the number of the months related to the cold season in stations were variable from three to five months which they are included in November, December, January, February, March and April. Although Zanjan the coldest and Gilvan the warmest are among the stations of the range under study, it is observed that there are two seasons in these stations and it should be noted that the seasons variation from hot to cold occur in more humid stations of a gradual coast and in the longer term, but the same event for the stations that are far from coast and are higher occur in a shorter period.