In this paper for the study of the amount of vulnerability of Iran to desertification we have used bioclimatic index of UNEP (United Nation Environment Programme). First average annual temperature and total annual precipitation has been collected from 45 synoptic meteorological stations for a 30 years period (1976-2005) from the Meteorological Organization of Iran. Then UNEP index are computed for all of studied stations. Using Mann-Kendall graphical–statistical test we then investigated UNEP changes trends in Iran.

On the basis of UNEP index we observed that most areas of Iran have arid and semi-arid climates. With respect to the desertification intensity class, these two kinds of climates have classes of severe and very severe conditions. After those two kinds of climates, ultra arid, dry sub-humid, very humid and sub-humid climates cover most areas in Iran respectively.

Results of Mann-Kendall test show that in three stations of Birjand, Tabriz and Mashhad, UNEP index changes trend have signs of tendency from semi-arid climate to arid climate. Changes trend at two stations of Urmia and Khoy have a tendency from dry sub-humid climate to semi-arid climate. Changes trend at station of Sanandaj has a sign of replacement between semi-humid climate with semi-arid climate and Yazd station shows tendency to ultra-arid from arid climate.

However, Zabol station at north of Sistan and Baluchistan province has had positive trend, whose changes trends have been from ultra-arid climate to arid climate. Changes trend of all of these stations at level of are significant.