Abstract: AIM: To optimize the preparation with in-liquid drying method of sustained release microspheres of levonorgestrol-poly 3-hydroxybutyrate (LNG-PHB) with good anticonceptive effect using PHB as the biodegradable material. METHODS: Uniformed design was used to optimize the technology of preparation with good reproducibility. The appearance, particle size, CHCl₃ residue, drug content, drug release in vitro, stability and anticonceptive effect on mice of the obtained microspheres were examined. The formation of the drug microspheres was confirmed with differential thermal analysis. RESULTS: The average particle size was 64 μm with over 90% of the microspheres being in the range of 28.7～85.8 μm. The CHCl3 residue was lower than 0.001%, and the drug release behaviour in vitro could be descibed by Higuchi equation. The drug release T1/2 was prolonged 1.8 times compared with the original drug LNG. The microspheres were stable in 3 months and showed significant sustained release and anticonceptive effect in mice and much lower toxicity compared with the original drug. CONCLUSION: The technology of preparation was successful and the anticonceptive effect in mice was satisfactory with much lower toxicity as desired.