Abstract:PS Ⅱ electron transport activities, chlorophyll and carotenoids content, SOD activities and isozymograms of needles from shaded and exposed seedlings and trees were monitored on Changbai Mountains during winter 1987-1985 to illustrate the internal protective mechanism of Korean pine (Pinus koraiensis Sieb. Et Zucc.) trees against winter photo-oxidation damage. The results show that the inhibition of photosynthetic capacity measured as PS Ⅱ electron transport rates are nearly as deep as solarexposed seedlings, but trees accumulated more chlorophyll and carotenoids in autumn can maintain higher contents in winter. SOD activities show higher activities in trees than those in solar-exposed seedlings as well. However, no extra isozyme bands are showed for trees. The data indicate that the mentioned substances may be synthesized by needles during soil frozen period. The possible role of SOD as free-radical scavenger in cold resistance is discussed.