Silicon nitride (Si3N4) nanocrystals were synthesized at about 250 degrees C by a simple organic-inorganic reaction between CH3SiCl3 and NaN3. The yield of Si3N4 is no < 70 wt% based on the amount of precursor CH3SiCl3 used in the reaction and TGA analysis. X-ray diffraction indicates the formation of a mixture of alpha- and beta-Si3N4. Particles with size from 40 to 100 nm are dominant in the products examined by transmission electron microscopy. X-ray photoelectron spectroscopy gives an atomic ratio of Si:N around 0.75:1. The formation of nanocrystalline Si3N4 during the organic-inorganic reaction goes through an intermediate product of NaSi2N3, which is important for understanding the reaction mechanism.