The diameter of particles which adversely affects the yield has been shrinking as ULSI devices are more and more miniaturized. Ultrafine particles with diameters of 0.1 mu m or less have become important recently. Ultrafine particles of this type are expected to be difficult to remove. This study has established a method to evaluate ultrafine particle removal efficiency. Ultrafine metallic particles with diameters of several to several hundreds of nanometers were deposited on the Si surface using a gas deposition method. The removal efficiency of the ultrafine particles using various cleaning solutions was investigated. APM (NH4OH-H2O2-H2O) cleaning can remove 150 nm Au particles, but cannot remove ultrafine Au particles with a diameter less than several tens of nanometers. In addition, the Si surface becomes rougher when a DHF-H2O2 cleaning is performed to remove Au ultrafine particles. This is believed to be because noble metals such as Au, Ag, and Cu, which feature a higher electronegativity than Si, attract electrons from Si facilitating Si oxidation.