Silver nanoparticles with uniform diameters were obtained in dodecyl trimethyl ammonium bromide (DTAB) solution at room temperature under magnetic stirring through hydrazine reducing AgNO3. The products were characterized by XRD, SEM and UV-vis absorption spectra. Results show that the silver nanoparticles are not from the direction reduction of Ag+ by hydrazine, but the products of the reduced silver bromide which are generated early. As the reaction time increasing, silver bromide of uniform sphere-like structure is gradually converted into nano-polyhedron silver bromide, and into silver nanoparticles finally