A green method has been developed for the syntheses of polyimide nanoparticles (PI NPs) at high pressure in supercritical carbon dioxide (scCO(2)) instead of conventional organic solvent. Without any complicated separation process, PI NPs can be obtained by depressurization of CO2. This method allows preparation of size-controlled PI NPs (a few nm to 194 mn) simply by varying the temperature at a fixed pressure of CO2.