The purpose of this study was to evaluate ovalbumin (OVA) leakage pathways and to explore the mechanism of the surface-indented microparticle formation in the preparation of OVA-loaded microparticles. OVA-loaded poly (D,L-lactic-co-glycolic acid) (PLGA) microparticles were prepared by a water-in oil-in water (w/o/w) solvent evaporation method associated with varied NaCl (NaCl) concentrations and adjusted with urea at 1240 mOsm kg(-1) in the external aqueous phase. To evaluate dichloromethane (DCM)-related OVA leak-age. three stirring rates, 600, 800, 1000 rpm at 25degreesC were carried out during the solvent evaporation stage. Both DCM and OVA levels in the external phase medium and total dispersion were sampled and measured. The time course of particle characteristics was evaluated by microscopy or SEM photography. The surface adsorptive capacities of the prepared microparticles were measured by Using bovine serum albumin conjugated with fluorescein isothiocyanate (FITC-BSA). The findings were that the DCM-related OVA leakage accounted for similar to34%, of the total leakage. By combining NaCl in the external phase, a faster solidifying crust-like structure was formed as a barrier to remarkably reduce OVA loss and improve OVA content from 40.1 to 72.8 mug mg(-1). The yield and OVA content for formulations containing NaCl were much improved by the ionic effect, in addition to the osmotic effect. The total entrapment efficiency Was also highly increased from 43 to 72%. The formations of the crust-like surface structure of the inicroparticle were affected by entrapped drugs, salt content in the external phase and aqueous volume in the inner phase. A scheme was proposed to interpret the formation mechanism of the surface-indented microparticles. In comparison to the surface-smooth microparticles, the surface adsorptive capacities of the surface-indented microparticles were highly improved from 26.6 to 87.0%, determined by the adsorption of FITC-BSA.