The purpose of this study was to propose a new preparation method to fabricate insulin-loaded poly(lactic-coglycolic acid) (PLGA) microparticles satisfying protein loading, release profiles, burst release, and particularly stability of the encapsulated protein. Insulin-loaded microcapsules were produced by a single phase o/o solvent evaporation method. The characteristics of the microcapsules were determined by various methods: the surface morphology and size of microparticles by atomic force microscopy and scanning electron microscopy, insulin crystalinity and drug-polymer interactions by XRD, DSC, and FTIR, chemical integrity and aggregation of insulin using HPLC and SDS-PAGE, the protein secondary structure by far ultraviolet-circular dichroism (CD), the antigenicity activity of insulin with ELISA techniques. PLGA microparticles showed smooth surfaces with microcapsule. Encapsulation efficiency of 51% and constant insulin release rate with initial insulin burst release of 24% was obtained. Encapsulated and released insulin was in the intact form and it was dispersed in crystalline state in the polymer matrix. Ease of manufacturing under mild preparation conditions, high level of drug entrapment, desirable release pattern with relatively low initial burst effect and an ability to preserve protein structure are the advantages which are offered by the developed protein encapsulation method.