low temperature extremes is among the major stresses that adversely affect plant growth and productivity. Cold stress causes oxidative stress, physiological, morphological and biochemical changes in plant cells. Generally, low temperatures similar to salinity and drought exert their negative effects mainly by disrupting the ionic and osmotic equilibrium of the cell. Changes in climatic condition leading to more frequent extreme conditions will require adapted crop species on a larger scale in order to sustain agricultural production. Eucalyptus is a diverse genus of flowering trees (and a few shrubs) in the myrtle family, Myrtaceae . Members of this genus dominate the tree flora of Australia . The Eucalyptus genus contains more than 580 species and also large number of cultivars which are native to Australia. Large distribution and diversity of compatible Eucalyptus cultivars reflect the fact of ecological felexibility of Eucalyptus. Some eucalyptus cultivars can sustain hard environmental condition like high and low temperature, salinity, high level of PH and drought Chilling and freezing are intensively effective on crops with tropical and subtropical origin such as Eucalyptus and most of the time it can be lethal for them. In this study we tried to evaluate cold tolerance of 12 Eucalyptus genotypes by means of four different methods: Chlorophyll Fluorescence, Electrolyte leakage, Proline and Stomatal density. These cultivars include six cultivars which are planted in Iran and six imported native cultivars from Australia. Except for stomatal density recording, in other methods plants are exposed to five gradual temperature drop:0, -5, -10, -15 and -20 degree of centigrade and they remained in these temperatures for at least one hour. Experiment for measuring chlorophyll fluorescence was done as 5×12 factorial in compeletely randomized design with three replicates. This experiment showed that genotypes no. 18 ( Eucalyptus erythrocorys ) and 13 were the most resistant genotypes and 2 ( E. gunnii ) and 11 ( E.coccifera ) were more sensitive than others. Electrolyte leakage experiment also was done as a 5×12 factorial experiment through complete randomized design (CRD) with three replications. With regards to significant interaction between cultivar and temperature and low increase in electrolyte leakage from -5 to -10 , genotypes 18 ( E. erythrocorys ) and 17 ( E.macrocorpa ) are shown to be most tolerant genotypes and 2 ( E. gunnii ), 3 ( E.urnigera ), 14 , 15 and 16 with the more ionic leakage percentage showed to be more sensitive to low temperatures. Proline experiment was done in the form of 5×5 factorial through complete randomized design (CRD) with three replications. Results of this experiment approved that the more resistant genotype in relation to cold stress is 18 ( e. Erythrocorys ). In the stomatal density experiment the number of stomata under microscopic field were counted and the results showed that the 18 ( E. Erythrocorys ) and 17 ( E.macrocorpa ) genotypes had the maximum and 11 ( E.coccifera ) and 12 ( E.dalrympleana ) genotypes had minimum number of stomata under microscopic field (0.0605 mm 2 ). As a conclusion, a remarkable correlation was not obtained between the stomatal density and other cold stress measures. key word: Cholorophyll Fluorescence, Cold Stress, Ionic leakage, Proline, Stomatal density