Water loss, internal structure of stems and permeability of cell cytoplasmic membranes of five clones, i.e., those of 1-63, 1-69, 1-72, 1-214 and the "blanc de garonne" of Aigeiros poplars, have been studied to measure their cold resistance during dormancy period. Results show that clones differ greatly in water loss; the amount of water loss correlates positively with the increment of autumn shoots and negatively with the cold resistance ability,The cell cytoplasmic membrane is more permeable after being injured by lowtemperature before any morphological changes appear, which can serve as a basis fordetermining the cold resistance of the clones. The order of the cold resistance of thefive clones can thus be determined as follows: I -214>"blanc de garonne"> l-72>I -69> I -63, which are similar to the results from field tests.In addition, the development of xylem and pith of the clone exerts a great influence on cold resistance. Clones with the highest cold resistance have been found in clones with the widest xylem, smallest pith and most layers of cortex and those with narrower xylem, bigger pith and fewer layers of cortex are generally less cold tesistante.