Frequent frosts occurs during winter and spring in Khorasan (Iran), that causes serious damage to walnut, which necessitating the selection of resistant cultivars. Before introducing new genotype, their capacity for adaptation to different condition needs to be studied. Walnut production in Khorasan has been increasing through area expansion and new genotypes. In this area affective freezes seldom damages walnut trees. The minimum temperature on winter of 2008 was about -220C. The objective of this study was to evaluate cold hardiness in 12 various walnut genotypes (K21B5/13, K21B5/15, K21B6/17, K21B6/18, K28B2/5, K28B2/6, K23B5/13, K23B5/14, K21B3/8, K21B9/25, K28B3/8, K21B1/3) which planted in 1986 on seedling at the genotype collection of Khorasan Agricultural and Natural Sources Researches Center. The results on 12 walnut genotypes showed that K21B5/13, K21B5/15, K21B6/18, K21B3/8, and K28B3/8 genotypes damaged more than other genotypes, and K23B5/13, K23B5/14 and especially K21B9/25 genotypes, showed to be resistance to minus 22°C. Results showed straight correlation between ionic leakage and visual observation. Minimum ionic leakage was observed in K21B9/25 genotype and visual observation verified its cold hardiness. Flower buds in K21B5/15 genotype and vegetative buds in K21B3/8 genotype were damaged completely. Sexual buds were the most sensitive organs of tree. Annual shoot was damaged more than biennial shoot in trees.

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