Abstract: The increase of nitrogen deposition may change the underground carbon cycle and soil carbon pool, and thus influence the structure and chemical components of fine roots. A number of researches have been done on fine root dynamics and morphological structure, but little is known of effects of nitrogen deposition on fine root structure and components. In the Great Xing’an Mountains, northeastern China, four sampling sites were set in a Larix gmelinii forest in May, 2012, i.e., controlling site (CK,0 g/(m·a)), low nitrogen treatment (TL, 2.5 g/(m·a)), medium nitrogen treatment (TM, 5.0 g/(m·a)) and high nitrogen treatment (TH, 7.5·g/(m·a)). In the growing season in July, 2014, the complete root system was dug out, and cortical thickness, stele diameter, root diameter, ratio of stele to root diameter of first five orders, and tissue chemistry (N, C and P) under different nitrogen treatments were measured. The influence of nitrogen deposition on morphological structure and chemical component of fine roots was analyzed. It showed that, with the ascending root order, indices such asroot diameter, cortical thickness and stele diameter increased accordingly. However, there were significant differences in cortical thickness, stele diameter, root diameter and ratio of stele to root diameter in the same root order under different levels of N treatment, and root tissue chemistry also had significant difference in different diameter classes. In conclusion, nitrogen deposition may impact cortical thickness, stele diameter, root diameter, ratio of stele to root diameter of first five orders and tissue chemistry (N, C and P), thus influences the physiological functions of fine roots, and finally has impact on above- and underground carbon cycle of plants.