Abstract: The critical micelle concentration (CMC) of three kinds of polyoxyethylene-polyoxypro-pylene block copolymer-L64, AE21, AP221 has been measured by surface tension and benzopurpurine uptake. Results showed that L64 and AE21 form monomolecular micelles at low concentration and aggregated micelles at high concentration, while AP221 only forms aggregated micelle at low concentration. Measurement of CMC at different temperature showed that in aqueous solution temperature has little effect on the CMC of L64 for monomolecular micellization, but has marked effect for aggregated micellization which decreases sharply with increase of temperature. Heat of formation of aggregated micelle has a large value, of 140.6kJ/mol.