Steel slags are major by-products of the steel and iron industry. Particularly rich in metallic trace elements, these materials are massively stored in slagheaps and constitute as many potential nuisances for the environment and public health. Several studies [(Mendez et Maier, 2008), (Vamerali et al., 2010)] have shown the effectiveness of phytostabilization processes for rehabilitating sites contaminated by high levels of metals, both because of their sustainable and ecological aspects but also due to their low implementation costs. The slagheap studied in this work contains approximately 500 000 tons of slags spread over an area of about 4 ha. A previous study (Bourchardon et al., 2014) carried out on the same site has demonstrated that the use of "Composted Sewage Sludge" (CSS) as an organic amendment favors the installation of metallophytes herbaceous species naturally able to grow on this type of substrate. In this work we evaluated :1) the combined effects of CSS and inoculation of an AMF (Arbuscular Mycorrhizal Fungi) on vegetation in terms of biomass production as well as accumulation of metallic compounds in leaves and 2) the optimal amount of CSS in order to obtain the best rates of mycorhization.