Roots are important structural and functional components of forest ecosystems. The mortality and decomposition of tree roots are key sources of soil organic carbon and nutrients. However, there is little information on root decomposition of Mongolia oak (Quercus mongolica) and Asia white birch (Betula platyphylla) in Northeast China. We hypothesized: (1) the decomposition rate decreased with root diameter increasing for both species, because coarser roots contain less nonstructural carbohydrates and nitrogen (N); (2) for the roots with same diameter in the two species, root with higher concentrations of soluble sugars and N also decomposed faster; (3) after one year's decomposition, most of the soluble sugars were released, but the N was little released or even concentrated. A buried bag experiment for the coarse (>10 mm), middle coarse (5-10 mm), medium (2-5 mm) and fine (<2 mm) roots of the two tree species was conducted at Maoershan Forest Ecosystem Research Station. In May of 2011, 21 bags (80-mesh, each contains 6 g root samples) for each root diameter class of each species (in total 168 bags) were buried at 10 cm depth in the natural oak-birch mixed forest at the same site (mid to upper-slope). Three bags for each diameter class of each species were sampled monthly for one year (in June, July, August, September, October in 2011; April and May in 2012). The roots were washed, dried at 75 ℃ and weighted. The concentrations of soluble sugars and N were analyzed for the initial and the decomposing roots. All the three hypotheses were supported by our results. The root mass remaining rate showed a descending trend with time, which could be well fitted (with the R²>0.71) by the Olson negative exponential decay model (X_t/X₀=e^-kt, where X₀ is the initial dry weight, X_t is the dry weight remaining at the end of the investigation, t is the time interval and k is the annual decay constant). The coarse, middle coarse, medium and fine roots of the oak had an annual decomposition coefficient of 0.2928、0.2562、0.2928 and 0.3660, respectively, while the coefficients of those of the birch were 0.2196、0.3294、0.3660 and 0.4392, correspondingly. After one-year decomposition, 20%-33% of the mass was lost for roots with various diameters. This supported the first hypothesis that the decomposition rate decreased with root diameter increasing. The birch has higher concentrations of soluble sugars and N for the roots with same size, which was responsible for the higher decomposition rate for the birch. During root decomposition, N concentrations in all sizes of the roots increased, while the concentrations of soluble sugars exponentially decreased. N mass in the four root sizes of both species released or uptaken during different decomposition time with no consistent trend, but soluble sugar mass was always releasing during the study period. At the end of the study (one-year decomposition), all sizes of root of the oak, and the fine and medium roots of the birch were N source for the soil, but the coarse and middle coarse roots of the birch were N sink for the soil. More than 90% of soluble sugars released from the fine and medium roots of the oak and birch, and more than 80% of soluble sugars released from the coarse and middle coarse roots for the two species. This study provided fundamental data for the C/N cycling in the natural generated forests in Northeast China.