Environmental assessment is often required in the field of waste management and often takes the form of a Life Cycle Assessment (LCA) study. This study focuses on the identification of environmental impacts of Municipal Solid Waste (MSW) treatment via Mechanical Biological Treatment (MBT). The MBT is a waste treatment emerging in France and aiming to separate the different fractions of MSW in order to recover them into matter and energy. This type of treatment is promising but we have little return on its real environmental performances. This MBT plant is composed of a pre-fermentation step (Bio Revolving System (BRS)), an anaerobic digestion step followed by a composting step and a gas and liquid treatment step. The majority of data are collected in situ by measurements and matter balance. The functional unit is “to manage the annual amount of MSW produced by a typical community”. Environmental impacts are assessed via the CML 2001 method. The generated impacts are due to gaseous emissions during refusals incineration and anaerobic digestion for global warming, during compost spreading with the nitrogen leaching as nitrate for eutrophication and during gas treatment with ammonia emission for acidification. Toxicity and ecotoxicity are mainly due to refusals incineration and to heavy metals contained into the spread compost. Energy recovery due to refusals incineration and biogas, and matter recovery due to the spread compost contribute to avoid global warming, acidification and eutrophication. Therefore, MBT is promising under two conditions: (i) an improvement of gaseous emissions knowledge, capture and reduction during biological treatment (composting and digestion), (ii) a better understanding of the opportunities of matter and energy recovery to keep vigilance on the existence of potential rebound effects. However, the question of the relevance of the compost spreading remains because of the dissemination in agricultural soils of trace heavy metals.