We measured and calculated the carbon density of litter fall in natural secondary birch -mongolian oak forest in the north of Changbai Mountain and Zhangguangcai Mountain .The litter carbon density in 42-year-old birch forest was 2.477 t· hm-2· a-1, and 1.049 t· hm-2· a-1 for 20-year-old birch forest, with difference of 136.13%.Litter carbon density in 20-year -old Mongolic oak forest was 0.948 t· hm-2 · a-1 , and 2.686 t· hm-2 · a-1for the 50-year-old Mongolic oak forest , with the deviation of 183.33%, and with the significance ANOVA result of ( P0.05) although in quantitative terms .The average value of carbon density in litter organs showed outstanding difference (P<0 .05) with leaf, branch, skin, fruit, dung, and ovum in the proper order , and the value rose with the forest age .Seasonal trend of carbon density in birch and Mongolic oak forest was regular and was divided into three stages according to the characteristics of carbon density trend , that was litter fall in growing season , litter fall in stop grow-ing season and winter dormancy with the highest value in the stop growing season , and the lowest in the winter dormancy .%在长白山北部、张广才岭西坡，东北林业大学老山人工林实验站，测试与计算天然次生白桦林（ Betu-la platyphylla Suk．）和蒙古栎林（Quercus mongolica Fisch．ex Ledeb．）凋落物碳密度。结果表明：20年生白桦林凋落物碳密度为1．049 t· hm－2· a－1，42年生白桦林凋落物碳密度为2．477 t· hm－2· a－1，相差136．13％；20年生蒙古栎林凋落物碳密度为0．948 t· hm－2· a－1，50年生蒙古栎林凋落物碳密度为2．686 t· hm－2· a－1，相差183．33％；经方差分析，均相差显著（ P＜0．05），说明林龄是影响碳密度变化的主要因素。2种林分类型凋落物碳密度的变化，经方差分析均无显著（P ＞0．05），在数量上虽有小的差异，但在理论上没有变化。不同凋落物器官碳密度的平均值，经方差分析相差显著（ P＜0．05），从大到小，依次为叶、枝、皮、果、粪便、虫卵，随着林龄增大排序还会有变化。白桦与蒙古栎林分凋落物碳密度，随季节的变化而有规律变化；根据凋落物碳密度季节的变化特点，分为3个凋落期，即生长季节凋落期、停止生长凋落期、冬季休眠凋落期，停止生长期凋落量最多，冬季休眠期凋落量最少。
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