In this study, amphiphilic Janus-type polymers were synthesized via ring-opening metathesis polymerization (ROMP), multiple vicinal diol formation, and grafting of poly(ethylene glycol) monomethyl ether (mPEG). These amphiphilic polymers formed self-assemblies, which were a mixture of micelles and multimicellar aggregates, in water. By choosing suitable Janus-type polymers and irradiating an aqueous solution of polymers using a sonicator, either small micelles or large multimicellar aggregates were obtained selectively. Hydrophobic substituents controlled the aggregation-disaggregation behavior, leading to the formation of metastable self-assemblies by sonication. The formation of self-assemblies with a uniform size was affected by ultrasonic frequency, rather than power. In vivo optical tumor imaging revealed that the large-size multimicellar aggregates persisting for a long time in blood circulation slowly accumulated in tumor tissues. In contrast, the tumor site was rapidly, clearly visualized using the small-size micelles.