Abstract: While many amphiphilic block copolymers can self-assemble into micelles in selective solvents, it is unusual to further form a physical gel. Some of block copolymers composed of poly(ethylene glycol) (PEG) and some polyesters undergo a sol-gel transition in water upon heating, which afford a potential class of new materials as an injectable hydrogel in biomedicine. While this system has been studied for about 20 years, it still remains at the stage of laboratory research today. A crucial bottleneck of its clinical application is the lack of a general guideline for molecular design, which needs a clear illustration of the internal structure and the mechanism of the thermogelling system. Recently, Prof. Ding's group proposed a mechanism model at the molecular level to demonstrate the sol-gel transition of the thermogel. A new type of micelle, called semi-bald micelle, was first proposed as the precursor for thermogelling, and confirmed from both computer simulations and experiments. Besides, Prof. Ding's group demonstrated that the thermogel structure was a percolated micelle network with hydrophobic channels evolved from the semi-bald micelles. This findings can not only explain the interesting thermogelling phenomenon, but also possess guidance of molecular design of this type of new materials. The proposed mechanism might also be stimulated to understand other responsive systems and corresponding self-assembly dynamics.