Sunflower downy mildew is one of the important diseases of the main crop production areas in Iran. The use of resistant varieties and hybrids is an effective method to avoid its damage. The soil samples containing the pathogen isolates, Plasmopara halstedii, were collected, refreshed and mass-produced on susceptible variety Record, using whole seedling immersion method. Based on existence of a dominant race (race 100) of the pathogen in sunflower cropping areas, 97 sunflower genotypes including F3 off-springs of cultivars Cherneanka and Gabor were evaluated against the disease under controlled conditions. Fifteen days post inoculation; the treated plants were evaluated based on six qualitative disease characteristics including damping-off, sporulation on cotyledons, sporulation on leaves and cotyledons, stunt, leaf mosaic or chlorosis and deformation. They were then converted to quantitative means based on their importance in disease development. Totally, 8 genotypes were categorized as resistant and the rest were identified as moderate-resistant (2), moderate-susceptible (2), susceptible (33), and highly susceptible (52).