The polysilsesquioxanes (PSQ) having alkoxyethylamide groups were prepared from the silane coupling reagents, prepared by addition reaction of alkoxyethylamines to (3-isocyanatopropyl)triethoxysilane, by condensation under acidic conditions. In the obtained PSQs, those having bis(methoxyethyl)amide or bis(ethoxyethyl)amide group showed reversible thermoresponsive phase separation in an aqueous solution. The PSQs formed by the co-condensation of such silane coupling reagents, the feed mole ratio of which were changed, also showed the thermoresponsive aggregation at the temperatures reflected with the ratios of alkoxyethyl groups.