In our continuing work to investigate Thai medicinal compounds derived from plants that exhibit anti-MRSA activity, we found that the ethanol extract of the nutgalls had the ability to inhibit and eradicate all clinical isolates of MRSA. The high amounts of tannins (20%) found in the nutgalls implied that tannins could be the active compounds responsible for the anti-MRSA activity (Kaur et al. 2008). The antibacterial, antifungal and antiviral properties of plant-derived tannins are well-known, but there are very few reports on their antimicrobial mechanisms (Singh et al. 2003). Thus, the effects of the nutgall extract as well as the main constituents on MRSA cell has been investigated and discussed in this report.