Drought is one of the most common natural events that have a great negative impact on agriculture and water resources. Recently, a new index for drought assessment and monitoring is presented called Reconnaissance Drought Index (RDI). RDI is calculated based on precipitation and potential évapotranspiration. In this study, indices of SPI (Standardized Precipitation Index) and RDI were calculated using 29 years meteorological data (1981-82 to 2009-2010) for seven regions. Correlation analysis is performed to identify differences of the SPI and RDI in different time scales. In order to assess the drought characteristics, NDVI indice was extracted in the 2003-2010 period, using MODIS images. So, the multi scales RDI from 1 to 12 months were correlated with NDVI indice. The Results showed that dry and wet periods on short time scales, in addition to precipitation depend on the evapotraspiration and the other weather parameters. It is recommended, in short time scales (1,3 and 6 months) to be used the RDI index for drought assessment. The rain effect on vegetation is less for dena and bouyerahmad areas (9 and 12 months), while it is high for Bahmaei and kohgilouye (3 months). So it is suggested that in range lands, 6-month time scale of RDI, in forest areas, long term time scales (9 and 12 months) and in rainfed lands, short term time scales (less than 3 month) can be used for drought monitoring. Copyright © EM International.