Abstract: Five kinds of natural medium and five kinds of synthetic medium were selected. Each medium was dumped into 20 conicalflasks. Each media had two treatments. In the first treatment, the natural media and synthetic media were sterilized for 15 min under maintaining pressure and naturally cooled until zero pressure. In the second treatment, the natural media and synthetic media were sterilized for 15 min under maintaining pressure and then the pressure was decreased to zero by releasing steam. All sterilized media were incubated in an incubator to observe contamination. The results showed that natural depressurization is suitable for sterilization of natural medium, while releasing steam is suitable for sterilization of synthetic medium.