【Objective】 The objective of this research was to investigate effects of long-term fertilization on formation of soil aggregates and the fraction of soil organic nitrogen therein in fluvo-aquic soil in the North China Plain, and consequently nitrogen supplying capacity of the soil and its mechanism.【Method】 Undisturbed soil samples were collected from the four treatments, i.e. CK (no fertilization), NPK (Application of chemical fertilizers only), 1/2OM (Application of half the rate of chemical fertilizer in Treatment NPK and half of the rate of organic manure in Treatment OM) and OM (Application of organic manure only) of the long-term (27 years) fertilization experiment at the State Agro-Ecological Experimental Station in Fengqiu, for analysis of contents of mechanically-stable soil aggregates and organic nitrogen therein, with the dry-sieving method and the Bremner method, separately. 【Result】 Results show that application of organic compost increased the proportion of > 2 mm aggregates in the topsoil, significantly or by 33% and 17% as compared with CK and NPK, whereas it decreased the content of <0.25 mm aggregates. Long-term fertilization significantly increased the content of organic nitrogen in aggregates in the fluvo-aquic soil, especially Treatment OM. The content of organic nitrogen in aggregates >2 mm, 2~0.25 mm and <0.25 mm was 776.4 mg·kg^-1, 837.7 mg·kg^-1 and 625.3 mg·kg^-1, respectively. The organic nitrogen in soil aggregates was dominated with acidolyzable ammonium N, which was followed by amino acid nitrogen and unknown-acidolyzable nitrogen, aminosaccharide nitrogen, the least. Treatment NPK increased the proportion of acidolyzable nitrogen in > 2mm aggregates, while Treatment OM did the content and proportion of amino acid nitrogen and unknown acidolyzable nitrogen. 【Conclusion】 long-term application of organic manure can improve the soil structure of fluvo-aquic soil significantly, and stimulate accumulation of total nitrogen and all fractions of organic nitrogen, with amino acid nitrogen, amico sugar nitrogen and non-acidolyzable organic nitrogen accumulated mainly in 2~0.25 mm, and acidolyzable and unknown acidolyzable nitrogen in >2 mm aggregates, thus significantly increasing nitrogen supplying capacity of the soil.