Drought as one of the most complex natural disasters is a creeping phenomenon. With the long period in its occurrence, it gradually appears to cause damages in different sectors such as agriculture, social, etc. Several indices are used to identify drought in accordance with their relevant assumptions. One of these indices is Palmer Drought Severity Index (PDSI) which uses hydrologycal and meteorological variables to solve a simple water balance equation in the soil. It considers the drought or wet conditions as dynamic phenomenon. The main goal of this study was to assess the PDSI based on its original assumptions, its regionalized status; and using the outputs of already calibrated and validated hydrological SWAT model in central regions of Iran. The PDSI in five methods was calculated. These are: 1) original Palmer Index without calibration in which the climate coefficients and the severity equation derived for Kansas and central Iowa; 2) original Palmer Index in which the coefficients of severity equations were adjusted; 3) the Palmer Index with the calibration of equations in central areas of Iran; 4) the Palmer Index using the soil moisture and potential evapotrairation from SWAT model; and 5) the Palmer Index using the soil moisture, potential evapotrairation and runoff from SWAT model. These methods were conducted for 17 major basins covering entire country with a monthly time step for the period 1990-2002. Further the drought severity equations were extracted for dry and wet conditions. Then these methods were applied to account for the PDSI in 160 sub basins in central Iran. The results showed that dual coefficients of severity equations for drought and wet years were 0.912- 0.13 and 0.93- 0.047 in method 2; 0.892- 0.002 and 0.946- 0.002 in method 3; 0.909- 0.011 and 0.945- 0.008 in method 4; and 0.894- 0.009 and 0.922- 0.007 in method 5, respectively. Though the coefficients were 0.897- 0.333 for both drought and wet conditions in method 1. The overall results of this study indicated that methods 2 and 4 had good agreement with method 3. Although the method 4 might address more reliable results as it is based on a robust accounting of physical processes. Therefore it can be expected that in the case of inconsistence with method 3 (10%), the method 4 provides more acceptable results. The results of our study agreed well for the years 1992 and 2001 for which the SPI, I indices recognized them as the most wet and dry years during the study period, respectively. Apart from the agreement that the Palmer Index showed with SPI and I, due to the dynamical nature of PDSI it can also detect the start and end of drought period. The approach used in this study is applicable for regional calibration of Palmer Index and using the outputs of other hydrological models. In addition this research study lays the basis for the assessment of the impacts of climate change on droughts in the future. Keywords : Palmer Index, Drought, Calibration, SWAT model, Central Iran.