A new device to study the thermophoresis of particles is presented in this paper. The instrument, called a radial flow thermophoretic analyser, is based on the geometry of the radial flow differential mobility analyser RF-DMA or SMEC (Spectromètre de Mobilité Electrique Circulaire). The device uses two circular plates, each with hot and cold temperature imposed. The instrument was designed with computational fluid dynamic software in order to check uniformity of the temperature field in the selection area after an establishment length. The transfer function of the instrument is then established and determines the effective thermophoretic velocity of particles selected in the temperature field imposed in the apparatus. For the experimental validation, effective thermophoretic velocities of monodispersed spherical latex particles for diameters ranging from 64 nm to 500 nm and a temperature gradient equal to 50,750 K/m are measured and compared with theoretical values. The results show good repeatability and discrepancies with theoretical values less than 7%.