The aim of this study was to encapsulate an anti-diabetic drug (Metformin hydrochloride) within poly (lactic acid) microspheres, by using a double emulsion solvent evaporation method with different emulsifiers. Response surface methodology using Box-Behnken design was used to optimize the effects of fours factors (the amount of metformin in the inner aqueous phase (X1), pH of the external aqueous phase (X2), amount of polyvinyl alcohol as a surfactant in the external aqueous phase (X3) and the stirring rate (X4)) on the encapsulation efficiency, particle size and zeta potential. The optimized microspheres hada spherical shape and exhibited zeta potential of −20.8 mV, average diameter of 271.41 μm and encapsulation efficiency of 78.05%. These values were reached by applying the following conditions: X1 = 25 mg, X2 = 4, X3 = 1.5% and X4 = 400 rpm. Differential scanning calorimetry and powder X-ray diffraction studies revealed that Metformin was present in an amorphous state in the microparticles. The study of the in-vitro drug release performed in simulated gastric and intestinal fluids showed that the drug release was more rapid with HPMC than with Span® 80 emulsifier.