Drought is a gradual phenomenon, slowly taking hold of an area and tightening its grip with time. Sometimes, in severe cases, drought can last for many years and can have devastating effects on the socioeconomic, agricultural, and environmental conditions that may result from one or more of the water-scarcity factors by insufficient precipitation, high evapotranspiration, and over-exploitation of water resources. Iran is one of semi-arid countries in the southwest of Asia where the drought-events cause some ecological and social-economic difficulties. This investigation aims to characterize the intensity and variation of drought periods from 1951 to 2005 over Iran through an integrated drought index (Palmer Drought Severity Index, i.e. PDSI). The obtained results have demonstrated, the drought period of 1999 – 2003 as the most intense drought event, and the northwestern regions of the country are relatively more vulnerable than the other parts.