Field experiments were conducted to study nitrogen composition of soil aggregates under straw and plastic film mulching in dryland wheat fields. Based on an 8 years mulching experiment in the Losses Plateau, the experiment set up three treatments:straw mulching (SM), plastic film mulching (PM), and no mulching (CK) during the growth period. The dry sieve method was used to determine the distribution characteristics of aggregates and the contents of total nitrogen (STN), microbial biomass nitrogen (MBN), and potential mineralizable nitrogen (PNM) in difference particle size aggregates. The results showed that:(1) Compared with no mulching, both straw mulching and plastic film mulching had no significant effects on soil total nitrogen content of difference particle size aggregates. SM increased the content of STN of the 1.00~0.25 mm particle size aggregate in the 0-10 cm soil layer compared with PM (12.88%, P<0.05). (2) Compared with CK, in the 0-10 cm soil layer, SM increased the content of MBN of the particle size classes >2.00, 2.00~1.00, and <0.25 mm by 18.67%, 24.05%, 20.08% (P<0.05), and increased the content of PNM of the particle size classes >2.00, 2.00~1.00 and < 0.25 mm by 35.13%, 30.03%, 42.88% (P<0.05). In the 10-20cm soil layer, SM increased the content of MBN of the particle size classes > 2.00 mm by 23.02% (P<0.05), and SM improved the PNM content of the particle size classes >2.00, 1.00~0.25 and < 0.25 mm by 28.59%, 31.31%, 32.48% (P<0.05). (3) Compared with CK, PM significantly increased the content of PNM of the particle size class < 0.25 mm in 0-10 cm soil layer (32.34%, P<0.05). (4) The content of nitrogen components of micro-aggregates (< 0.25 mm) was significantly higher than that of macro-aggregates (> 0.25 mm), however the contribution rate of the macro-aggregates nitrogen component was 81.88%~87.66%. SM increased the contribution rate of soil macro-aggregates to nitrogen component, so that more nitrogen was stored in macro-aggregates, and the effect of PM on the contribution rate of nitrogen in the aggregates was marginal. Overall, compared with CK and PM, SM increased the STN content and the nitrogen content of the micro-aggregates and macro-aggregates, so that more nitrogen is stored in the macro-aggregates and promote soil nitrogen turnover.