This study was systematically designed to compare bovine serum albumin (BSA) loaded poly(epsilon -caprolactone) (PCL) microparticles based on a 2(3) factorial experiment. The microparticles were prepared by the holt-melt technique without using an organic solvent for polymer solubilization. The influence of the particle size of protein, protein/polymer ratio, and hydrophilic PEG on the surface morphology, particle size as well as the yield of PCL microparticles, encapsulation efficiency of BSA, and in vitro release properties were investigated. The structure of BSA remained its integrity using this technique. The mean particle size of BSA-loaded microparticles were in the range of 12.7 +/-0.1-16.9 +/-0.8 mum, and all of the particles were smooth on the surface. The production yield of microparticles and the encapsulation efficiencies were high, and the values were in the range of 94.8 +/-1.6%-98.1 +/-1.0% and 94.9 +/-9.6%-98.6 +/-0.3%, respectively. The burst release of BSA was in the range of 8.2 +/-0.4%-61.0 +/-0.8%, which strongly depended on the formulation. None of three variables affected the yield of microparticles prepared from eight formulations (p > 0.05). However, the particle size of BSA significantly affected the size and the burst release as well as the cumulative release of protein in these microparticles (p < 0.05). The initial loading of BSA in terms of BSA/PCL ratio and the amount of PEG blended with PCL significantly affected all of the properties, except the yield (p < 0.05). The smaller the particle size of the BSA, the smaller the size of the resulting microparticles. Since the total surface area of the small particles was larger than that of the large particles, this accounted for the high burst release of protein from the microparticles encapsulating triturated-BSA.