PURPOSE: To permit particles to be classified into more ultra-fine particles than attained by the conventional dry classifier by a method wherein, in a dry classifier for classifying the particles while carrying the same by a gaseous stream, the gaseous stream consists of the gas lower in viscosity than the air at normal temp.

CONSTITUTION: In a dry classifier A for classifying particles while carrying the same by a gaseous stream, the gaseous stream consists of the gas lower in viscosity than the air at ordinary temp. In addition to cooled air, there are used as the aforesaid gas at least one kind of the gases or vapors selected from hydrogen, nitrogen, methane, ethane, propane, normal butane, normal pentane, normal hexane, normal heptane, normal octane, ethylene, propylene, carbon tetrachloride and chlorofluorocarbon. When the dew point of the aforesaid gas is not higher than its normal temp., it is a low temp. gas lower in viscosity than the normal temp. air. As a result, this method permits the particles to be classified inexpensively into more ultra-fine particles than attained by the conventional dry classifier.