Poly(D,L-lacic acid)(PLA) microshperes containing loazepam were prepared by a solvent-emulsion evaporation method and their release patterns were investigated in vitro. Various batches of microspheres with different size and drug content were obtained by changing the ratio of lorazepam to PLA, PLA concentration in the dispersed phase and stirring rate. Rod-like lorazepam crystals on microsphere surface, which were released rapidly and could act as a loading dose, were observed with increasing drug content. The release rate was increased with increase in drug contents and decrease in the molecular weight of PLA. The release rate of lorazepam for long-acting injectable delivery system in vitro, which would aid in Predicting in vivo release Profile, could be controlled by properly optimizing various factors affecting characteristics of microspheres.