Nano-sized self-assemblies with various morphologies are being extensively studied to use them in a variety of biological and industrial applications including targeted drug delivery. This study reports a novel strategy to prepare selfassemblies with high aspect ratio by varying packing structure. We synthesized poly(2-hydroxyethyl aspartamide) (PHEA) grafted with porphyrins, which could form self-assemblies in an aqueous solution. Then, increasing the degree of substitution of porphyrins induced the structural transition to rod-like assemblies with an aspect ratio of 10 by inter/intra molecular π-π stacking of porphyrins. Further introduction of metalloporphyrins to PHEA leads to an uptake of oxygen molecules. This strategy strategy to prepare polymer self-assemblies will serve to improve the efficiency of targeted delivery for a molecular optical and ultrasound imaging with various biomedical modalities.