The comparison data of frost hardiness evaluation of the initial plum forms in a laboratory by direct freezing method are given in the article. 7 varieties and 2 promising hybrids of domestic plum tree and 6 varieties and 2 promising hybrids of diploid plum tree were the objects of the investigations. Winter beginning is a critical period enough for a plum culture. Besides diploid plum has a higher level of frost hardiness at the very period. At the end of winter vice versa diploid plum variety has a more negative reaction on subzero temperatures than domestic plum varieties. It is determined that the weakest points of plum frost hardiness are tissues of vessel xylem which directly feed the buds. Their significant damage was observed at all stages of the research. Low subzero temperatures at the initial winter period have a more negative impact on tissues of wood xylem. Meanwhile, at the winter end the lower frost hardiness is at tissues of bark phloem. The etalons of high frost hardiness by all three components of winter hardiness appeared to be the varieties of domestic plum tree ‘Dalikatnaya’ and ‘Perdrigon’ and diploid plum variety ‘Mara’.