The microstructure and bond character of the nanometer-sized amorphous silicon nitride solid (NANO-SNS), heat-treated in the temperature range of 25°-1000℃, were studied in terms of X-ray radial distribution functions. It was observed that the interfaces with a large volume fraction were not the "gas-like" structure but the short-range order structure, which is different from that inside the amorphous nanoparticle. Both Si-N bond length and coordination numbers of the first nearest neighbours(CN) in interfaces are less than that in traditional Si3N4 and there exist a large number of Si dangling and unsat-urated bonds. NANO-SNS has an nonty-pical covalent structure that contains lots of unsaturated and dangling bonds. This bond structure is different from the saturated covalent bond structure of traditional Si3N4.Due to the unsaturated character of the bond coordination numbers, the molecular form of NANO-SNS should be written as Si3-xN4-y. The appearence of strong polarity of NANO-SNS is closely related to unsaturated and dangling bonds.