Posidonia oceanica in one of the most important endemic species of Mediterranean Sea. In pristine coastal seawaters, it forms extensive meadows on mobile and hard substrata. Posidonia beds provide a lot of ecological roles and, amongst them, there is new ecosystem service known as “Blue Carbon”. This important function is the potential, performed by the meadows, to reduce the increasing concentration of carbon dioxide in the terrestrial atmosphere, binding it as organic matter in the leaves of the plants and, also, in the sediment of this valuable ecosystem. This study aims to assess the process of carbon sequestration performed by Posidonia beds and the dynamics of carbon storage through the different levels of the meadows.