Aromatic plants show antimicrobial activity due to their essential oils, but their effect on litter decomposition is unclear. In this study, we evaluated the biomass loss and nutrient dynamics in leaf litters of two macrophytes (Miscanthus sacchariflorus and Carex brevicuspis) with and without addition of powdered material of the aromatic plant Polygonum hydropiper or the non-aromatic plant C. brevicuspis. The two powders had similar basic chemical qualities but P. hydropiperi had a higher essential oils concentration. Leaf litters of M. sacchariflorus and C. brevicuspis were incubated with powdered P. hydropiper or C. brevicuspis (500 g m-3, 250 g m-3, and no addition) for 120 days in a mesocosm experiment. Compared with the control (no addition), P. hydropiperi addition decelerated nutrient release and litter decomposition, while C. brevicuspis addition accelerated those processes. The nitrogen concentrations in both leaf litters and the phosphorus concentration in C. brevicuspis leaf litter were increased by addition of both plant powders. The fungal biomass in both leaf litters decreased after P. hydropiperi addition, due to the antifungal activity of its essential oils. These data indicate that the aromatic plant P. hydropiperi inhibits litter decomposition via its essential oils and that such inhibition is not species-specific.