Poly(_DL-lactide-co-glycolide)(90:10) was synthesized from _DL-lactic acid and glycolic acid and The copolymers was made to microspheres containing clonazepam for drug delivery system. The release experiments of the drug from micropheres were operated at pH 7.4 phosphate buffer solution 37.0±0.05℃ and these results were as follows. The linearly-releasing time ranges of the drug from microspheres prepared with the copolymers/drug weight ratio of 20:40, 20:20, and 40:20 (mg) were 42, 34, and 22 days, respectively. So the linearly-releasing time of drug showed the order of microspheres 20/40 > microspheres 20/20 > microspheres 40/20. In short, the formulation allows polymeric microspheres to suppress the burst effect of the drug release mechanism, which led to the controlled release pattern and tole possibility of DDS for veinous injection.