Mechanism of the morphological changes between toroidal and rod-like nanostructures of P4VP-b-PS-b-P4VP amphiphilic triblock copolymer micelles has been investigated in aqueous solution. This transition is proved to be highly reversible and tunable upon changing temperature. The toroidal structure, evolving from fibers at 20 degrees C, can transform to rod-like morphology as the temperature either gradually or directly increases to 80 degrees C, and vice versa. However, the transition mechanisms are quite different in different temperature-changing processes. The structure and thickness of the micelles are dependent on the specific temperature, whereas the transition mechanism is related to the method of the temperature change. These morphological changes are considered as a result from the interaction parameter between the solvent and the copolymer blocks, especially the hydrophobic block. Our research complements the external control over the reversible morphological transition of block copolymer micelles without changing the composition of the system or introducing additional influencing factors. (C) 2016 Wiley Periodicals, Inc.