Poly(alpha-hydroxy octanoic acid) was first used as an additive for the preparation of electrospun ultra-fine fibers of poly(ethylene glycol)-b-poly(L-lactide) (PEG-PLLA). Ibuprofen was loaded in the electrospun ultra-fine fibers. The results from environmental scanning electron microscopy (ESEM), wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) demonstrated that ibuprofen could be perfectly entrapped in the fibers electrospun from PEG-PLLA using alpha-hydroxy octanoic acid or PEG-b-poly(alpha-hydroxy octanoic acid) (PEG-PHOA) as additives. Compared with electrospun PEG-PLLA fibers which entrapped 20 wt% ibuprofen, the PEG-PLLA electrospun fibers containing PEG-PHOA exhibited integral and robust after 1 week incubated in 37 degrees C, pH 7.4 phosphate buffer solution with 10 mu g/mL proteinase K. Compared with electrospun fibers without PEG-PHOA, the concentration of proteinase K in release media had less effect on the release rate of ibuprofen. An unique release profile was found from PEG-PLLA fiber after the incorporation of PEG-PHOA. Enzyme degradation experiments demonstrated that PEG-PHOA but not alpha-hydroxy octanoic acid monomer was the crucial factor for integrity maintenance of the electrospun fibers, which may be due to the enzyme degradation tolerance property of the PEG-PHOA polymer additive.