We introduced newly biodegradable polymer surfactant to poly(D,L-lactide) microspheres for exhibiting precisely controlled release rate of antitumor agent. We newly designed biodegradable polymer surfactant, poly(ethyleneoxide monooleate)-block-poly(D,L-lactide); (MOPEO-PLA), which has a biocompatible, oil-soluble and compatible with polylactide matrices. Polylactide (PLA) microspheres with MOPEO-PLA enclosing Irinotecan hydrochloride (CPT-11) was successfully fabricated by solvent evaporation method via O/O emulsion. The surface morphology was strongly affected to MOPEO-PLA content in PLA microspheres; the wrinkles on microsphere surface were decreased with increasing MOPEO-PLA content. It was found from DSC scans that Tg of PLA drastically decreased with introducing MOPEO-PLA, indicating that MOPEO-PLA miscible with PLA. MOPEO-PLA, played as a plasticizer, made the surface smoothed. The rate of release in a short duration was minimum value at adding 20 wt.% of MOPEO-PLA. This result was due to the smaller surface area of PLA microspheres with smoothed surface.