Potato cultivars differ in their susceptibility to stem canker and black scurf disease caused by Rhizoctonia solani. Cultivar reactions of the most nine common commercial cultivars of potato were tested under greenhouse conditions of plant pathology research Institute, ARC-Giza. The nine tested cultivars were divided to three groups . Lady Rosetta was the most susceptible cultivar followed by Monalisa and Mondial cvs. Spounta , Nicola and Hernez cultivars showed moderate susceptibility , While Draga followed by Kara and Diamond were the most resistant . The two potato cultivars Draga the resistant cv.and lady Rosetta , the highly susceptible one were selected to study the nature of disease resistance among potato cultivars . Fraction of phenolic compounds by HPLC, oxidative enzymes and thio amino acid contents were determined in leaves and root of these potato cultivars. The resistance cultivar Draga showed higher amounts of free , conjugated and total phenols , fraction of phenolic compounds ( Coumarin ,Caffeic acid,p-Coumaric acid ,Benzoic acid ,Resorcinol and Apigenin ), oxidative enzymes ( peroxidase and polyphenoloxidase ) and thio amino acids ( Cysteine , Cystine and Methionine ) compared with lady Rosetta , the highly susceptible cultivar.