Sixty isolates of Pythium aphanidermatum as the causal agent of sugar beet root rot were selected on the basis of their geographical origins, morphological and genetic diversity studies. Pathogenicity test using the Kruskal-Wallis analysis showed significant differences between the pathogenic potential of isolates. Based on the growth rate, isolates were categorized in two groups with low and high growth rate. The potential of ISSR and rep-PCR for fingerprinting purposes was evaluated using eight ISSR and three rep-PCR primers. A strong linear relationship was observed between resolving power of a primer, PIC, and MI. PIC value for the rep-PCR primers (0.41) was higher than the ISSR primers (0.39). Based on UPGMA method and Jaccard's coefficient, ISSR separated Khuzestan and Fars provinces isolates from other isolates at 63% similarity level, while rep-PCR separated these isolates at 61% similarity level. The PCA analysis showed that despite the similarities of both markers, rep-PCR represent a higher resolution than the ISSR. So the rep-PCR marker leads to further differentiating of the isolates. There was a relationship between genetic divergence and geographical origins of isolates. rep-PCR can replace ISSR in diversity studies because of their comparable accuracy in the interpretation of genetic diversity of isolates.