One of the most important ways to reduce atmospheric carbon is the carbon sequestration by trees. Chestnut-leaved oak (Quercus castaneifolia C. A. Mey) is one of the most important native oaks of Iran distributed in the Hyrcanian Forests. The pure and mixed stands of it cover about 6.5% of these forests. In this study, carbon sequestration of chestnut-leaved oak seedlings was evaluated by using some morphological characteristics of the root and shoot. For this purpose, one hundred seedlings were sampled by method of Systematic-Random from the sowing bed on March 2022 in the Pylambra nursery at Guilan province. Seedlings are divided to three grades small, medium and large according to Root Collar Diameter (RCD). The biomass and carbon sequestration of chestnut-leaved oak seedling were calculated according to the basic density of its root and shoot. The Pearson's correlation coefficient was used for correlation detection between variables. The one-way analysis variance test at the 95% confidence level was used to recognize difference among biomass and carbon sequestration of three group of the oak seedlings. The results of correlation analysis showed that the root collar diameter (RCD) had the strongest correlation with other morphological characteristics. The amount of the basic density for the root and shoot of the oak seedling was obtained about 0.57 g/cm3 which is the same for both of them. The amount of the biomass and carbon sequestration of the root was obtained more than shoot at the small and medium seedlings, whereas in large seedling was the same. In general, by increasing the size of seedling the biomass and carbon sequestration increased.