Poly(ethylene glycol)-block-poly(partially benzyl-esterified aspartic acid), denoted by PEG-P(Asp(Bzl)), is one of the most examined blockcopolymers for drug carriers. However, little is known about fundamental physical properties. Nine samples of PEG-P(Asp(Bzl)) with different benzylation fractions (F-Bzl) and aspartic: chain lengths (DPAsp) were synthesized, and the aggregation number (N-agg), core radius (R-C), and other structural parameters were determined with combination of light scattering and synchrotron X-ray small-angle scattering. The major factor to determine N-agg and R-C was found to be F-Bzl,, i.e., the hydrophobic nature of the core, even though F-Bzl was changed in the relatively small composition range from 66 to 89 mol %. When we compared the data for the same F-Bzl, the scaling theory was consistent with the core chain length dependence of both core and micelle sizes. The overcrowding nature of the tethered PEG chains on the micelles was increased about 1.3-2.9 times with increasing N-agg compared with the unperturbed state in solutions.