Abstract: Based on the theory of coupled vibration and flexural vibration of a long thin bar with rectangular cross section, we studied the flexural vibration of rectangular plates, derived out the frequency equation when the boundaries of plates are free, studied the relation between resonant frequencies and vibrating modes. Experiments show that the measured results of resonant frequencies agree well with their calculated results, the ultrasonic cleaning tank with the rectangular plate of flexural vibration as ultrasonic radiators has the advantages of uniform acoustic field, large acoustic radiating surface and easy adjustment of exciting frequencies over traditional ultrasonic cleaning tank, the rectangular plate of flexural vibration proves to be a promising high power ultrasonic radiating source.