Well-dispersed Silver colloids are prepared from AgNO3 and KBH4 under the protection of polyvinylpyrrolidone (PVP) in the ultrasonic field. The main factors influencing on stability of silver colloids, such as the concentration of PVP and KBH4, ultrasonic time and power, are discussed. The silver colloids are characterized by TEM, SAED and Spectrophotometer. The result indicates that the stable silver nanoparticles are prepared with PVP/ AgNO3 mass ratio of 1:1. Selected area electron diffraction pattern reveals that the silver nanoparticle is of face-centered cubic structure. Spherical-like silver nanoparticles with about 20 nm size are synthesized with AgNO3/ KBH4 molar ratio of 1:2. The dispersion and stability of the silver colloid will be improved after ultrasonic wave treatment. The diameters of silver nanoparticles decrease and the particle size distribution becomes narrow with the ultrasonic power increase. Silver particles with 10~20 nm size and spherical shape have been prepared after 50 min of ultrasonic wave treatment.