Doxorubicin, which is a positively charged anti-cancer drug, was encapsulated into the micelles formed by copolymers consisting of methoxy poly(ethylene glycol), negatively charged aspartic acid oligomer and poly(ε-caprolactone) (mPEG-Asp-PCL). The micelles showed an intensity-averaged diameter of 73.0 ± 30.6 nm measured by dynamic light scattering. The diameter of doxorubicin loaded micelles increased slightly to 75.8 ± 26.2 nm. The doxorubicin-loading and efficiency into the micelles were 15.1% and 44.5%, respectively. The drug-loaded micelles were stabilized ionically using divalent calcium cations and displayed a size of 78.1 ± 24.3 nm. The stabilized micelles showed the typical two-phase release patterns, which were the relatively rapid release of 53% doxorubicin in the first 24 h, and then showed the sustained release to 65% of more than 90 h. On the other hand, doxorubicin release from the non-stabilized micelles was retarded and did not exceed 5% in 24 h because the positively charged doxorubicin at pH 7.4 in the absence of cations was still retained in the micelles through ionic interactions with the carboxylic acids of aspartic acid residues. At pH 3.0, the carboxylic acids became protonated and the neutralization of the charges led to an absence or decrease in the ionic interactions, which resulted in the abrupt release of doxorubicin up to 41% from the non-stabilized micelles after a change in pH and steeper release profiles up to 59% in 90 h.