In order to investigate the genetic diversity and select the best drought tolerance index of the most feasible genotype of castor (Ricinuscommunis), 12 genotypes were evaluated in a split plot design with totally random block of tree repetitions under two levels of irrigation (stressed and stress free) at the research farm of ‘Uromiyeh Agricultural Research Center’ in 2009 farming year.To evaluate drought tolerant genotypes on the basis of yield performance in stressed (Ys) and non-stressed (Yp) environments, quantitative measures of drought tolerancesuch asmean productivity (MP), stress tolerance (TOL), geometric mean productivity (GMP), harmonic mean(HM), stress susceptibility index (SSI) and stress tolerance index(STI) were computed. In this respect and based on the correlation results of different indices and the reactions of genotypes in the two environments we observed that the STI, HARM, MP and GMP indices have the most correlation with the performance under the two conditions. The highest amounts of STI, HM,GMP and MP were related to the genotype 80-12-1 in both stressed and non-stressed conditions. Based on these four indices and the above reactions in both conditions the genotype 80-12-1 was selected as the drought tolerant genotype. Cluster analysis located tolerant genotype 80-12-1 in one group and the other genotypes in another group as susceptible genotypes.