The present invention relates to a composition for treating a bacterial infection comprising an extract from a bark of Alnus pendula, or diarylheptanoid compound as an active ingredient. The extract from a bark of Alnus pendula, diarylheptanoid compound of the present composition has a remarkably excellent anti-bacterial activity against bacteria, specifically Staphylococcus aureus. Therefore, the extract and compound of the present invention may be utilized as an active ingredient of drug, cosmetics, functional food and animal feed for treating bacterial infection, in particular an infection of S. aureus.