The Si3N4 nanopowder was prepared by means of ICPECVD by decomposing of Sill4 and N2.The ion density in the reaction chamber was diagnosed by a Langrnuir probe.Analysis by FT-IR, TEM and XRD reveals the characteristic of Si3N4.The results indicate that Si3N4 nanopowder synthesized by ICPECVD is hexagonal cell β-Si3N4 and the particles is spherical with diameter about 20nm, the distribution of those particles is symmetrical.