A new washing device was used to separate hydrophobic trimetal nanoparticles with particle size less than 10 nm monodispersed in organic solvent. The washing device and method is not only easy to control but also very efficient. Quantum dots with different sizes such as 4.5 nm, 5.1 nm, 6.4 nm were obtained. The UV-Vis spectra of those nanoparticles were at 522 nm, 529 nm, 538 nm indicating very good separation. Particularly, the absorbent peak of the particles with size of 4.5 nm is very narrow distributed. A new washing device was used to separate hydrophobic trimetal nanoparticles with particle size less than 10 nm monodispersed in organic solvent. The washing device and method is not only easy to control but also very efficient. Quantum dots with different sizes such as 4.5 nm, 5.1 The UV-Vis spectra of those nanoparticles were very narrow at 522 nm, 529 nm, 538 nm. Particularly, the absorbent peak of the particles with a size of 4.5 nm was very narrow distributed.