The self-assembly of dendritic-linear block copolymers PEG(G_m)-b-PS (where m is the generation number, and m = 1, 2, 3) of different generations with similar hydrophilic weight fractions in aqueous media was studied. With the increasing generation number, the self-assembled aggregates transform from polydisperse vesicles to a mixture of cylindrical micelles with a few vesicles. Turbidity measurements, transmission electron microscopy, and scanning electron microscopy were used to explore the effect of the solvent on the aggregation process in mixtures of tetrahydrofuran–water. Depending on the content of water added, many intermediate structures like large compound vesicles with porous surfaces or combined morphologies from vesicles and cylindrical micelles could be found. In situ monitoring of the intermediate structures provided mechanistic insights regarding their formation processes. For PEG(G₁)-b-PS and PEG(G₂)-b-PS, the results showed that the fusion and fission processes occur during the addition of water. The fusion process is composed of two steps. First, the vesicles contact and adhere to each other. Second, they rearrange to form larger structures. In the fission process, the large structures are distorted or elongated at first, and then they transform into smaller vesicles. For PEG(G₃)-b-PS, only the adhesion and separation processes were detected, without further fusion and fission. Comparing the aggregation processes induced by solvent of samples with different dendron generations, we propose that it is easy for the fusion process to take place between vesicles and difficult between vesicles and cylindrical micelles.