A plate-to-plate thermophoretic precipitator with high particle collection efficiency has been designed and tested. The precipitator consists of two flat plates separated by a 0.38-mm thermal insulating shim. The top plate is heated by a constant high-temperature water flow while the bottom plate is cooled by a constant low-temperature water flow. Through this forced con-vective heat transfer arrangement, a high and uniform temperature gradient is achieved between the two plates, where submicron aerosol particles can be collected efficiently and uniformly in the direction of the air flow. The precipitator was tested in the laboratory using NaCl and sodium fluorescein monodisperse test particles. When the temperature gradient ranges from 502° to 879°C/cm and the Knudsen number ranges from 0.27 to 3.5, the experimental data show that the present precipitator collects submicron particles efficiently. Among previous theories, the thermophoretic velocity equation proposed by Talbot et al. (1980) is found to fit very well with the present experimental data.