Increasing nitrogen (N) deposition significantly affects global forest ecosystems. It is not clear how N deposition affects carbohydrate contents of different diameter fine roots, which limits the understanding on the mechanism of plant physiology and growth affected by N deposition. We set up a N addition experiment with 2-year-old Pinus tabuliformis seedlings to examine the effects of different N addition levels on nonstructural carbohydrate (NSC) and structural carbohydrate (SC) contents in different diameter fine roots. Four N treatments were designed, including N0, N3, N6 and N9 (0, 3, 6 and 9 g N·m^-2·a^-1, respectively). The results showed that: (1) the carbohydrate contents (512.97 mg·g^-1) in fine roots with 0-0.5 mm diameter were significantly lower than those (638.83 mg·g^-1) in fine roots with 1-2 mm diameter. The NSC, soluble sugar and starch contents in the fine roots with 0-0.5 mm diameter were significantly lower than those in fine roots with 0.5-1 and 1-2 mm diameter; (2) The SC and the cellulose contents had an increasing trend with the increase of fine root diameter, while lignin content showed a decreasing trend. Different responses of carbohydrate components in different diameter fine roots to N addition were observed in this study. N addition had no significant influence on the NSC, soluble sugar and starch contents in the 0-0.5 and 0.5-1 mm fine roots, except that N9 treatment significantly reduced the NSC and soluble sugar contents in 1-2 mm fine roots (by 16.20% and 29.90%, respectively) and N3 treatment significantly increased the soluble sugar content in the 0-0.5 mm fine roots (69.65%); (3) Nitrogen addition had no significant influence on the SC and lignin contents. N3 treatment significantly increased the cellulose content in 0-0.5 and 0.5-1 mm fine roots (35.3% and 57.0%, respectively), while N9 treatment significantly reduced the cellulose content in 1-2 mm fine roots (30.39%); (4) N3 treatment significantly increased NSC/SC ratio in the fine roots with 0-0.5 mm diameter, but had no significant effect on those with 0.5-1 and 1-2 mm fine roots. Our results indicated that N addition may influence fine root physiological function and growth via affecting NSC, soluble sugar contents and NSC/SC ratio in the fine roots with 0-0.5 and 0.5-1 mm diameters.