The influence of the charged segment length on the physicochemical properties of polyion complex (PIC) micelles formed from the mixture of poly(ethylene glycol)-b-poly(alpha,beta-aspartic acid) [PEG-P(Asp)] with poly(ethylene glycol)-b-poly((L)-lysine) [PEG-P(Lys)] or poly((L)-lysine) [P(Lys)] was evaluated in this study. The hydrodynamic radius (Rh) and apparent weight-averaged molecular weight (M-w,M-app) Of the PIC micelles were determined from the dynamic and static light scattering to calculate the association number, core radius (R-core), micellar radius (R-m), corona thickness (R-corona), and PEG density at the core/ corona interface (PhiPEG). Changes in R-core and Phi(PEG) of the PIC micelles with a change in the charged segment length of the block copolymer were explained by considering the balance between the interfacial energy and conformational entropy of polymer strands associated in the core-shell type PIC micelles. Balance among these two parameters (R-core and Phi(PEG)) seems to play a substantial role in determining micellar size, the association number, and, thus, M-w,M-app of the micelles. Furthermore, Phi(PEG) was shown to depend only on the length of the P(Asp) segment in PEG-P(Asp), regardless of the composition of the counter polycations (PEG-P(Lys) and P(Lys)). This result suggests that Phi(PEG) might be the most crucial parameter to determine the physicochemical characteristics of the PIC micelles.