The use of levo-ornidazole in the preparation of medicine for preventing and treating the anti-anaerobic bacteria infection is provided. It is demonstrated that levo-ornidazole exhibits lower toxicity and less central inhibition effects than dextro-ornidazole or racemic ornidazole. L-ornidazole possesses pharmacokinetics characteristics, which are superior to that of the racemic ornidazole, and anti-anaerobic activities which are slightly better than or substantially the same as that of the racemic ornidazole. Also, this invention particularly relates to a preparation process, which comprises formulating L-ornidazole as anti-anaerobic infection pharmaceutical preparations, which are suitable for clinical uses.