Although several studies have assessed the efficacy of chlorhexidine in treating gingivitis and reducing biofilm formation, the side effects of a long-term use are unpleasant for patients. Melaleuca alternifolia oil has been considered as an alternative because of its anti-inflammatory properties. However, little is know about the efficacy of its nanoparticles. The purpose of this study is to compare the effect of 0.12% chlorhexidine (Periogard ®) and a nanoparticle solution of 0.3% M. alternifolia oil in reducing biofilm formation in plaque-free and plaque-covered teeth surfaces.