Composting has always been considered a way to optimize use of biodegradable wastes. The point which is perhaps less noticed is that this process is not necessarily always and everywhere economical. Energy consumption and greenhouse gas emissions are among factors that can impugn eco-efficiency of the process in composting factories. The present study is an eco-efficiency assessment of a composting factory in Tehran calculating the real economic and ecological efficiency of cthe omposting process by considering two main external burdens: energy consumption and greenhouse gas emissions over the period 2007-11. In this research, the instruction presented by the World Business Council for Sustainable Development [1] was followed up to quantify eco-efficiency indicators. The obtained results revealed that the composting factory experienced a negative energy eco-efficiency within the periods 2007-08 (-71.97%) and 2008-09 (-69.82%). In terms of CO₂ emission eco-efficiency, there were also inefficient periods in the plant in two time intervals: 2007-08 (-71.97%) and 2008-09 (-69.80%). Fortunately, this was a temporary situation so that at the ultimate year of 2011, the composting plant had an acceptable performance in terms of energy consumption (with an eco-efficiency of 166%) and CO₂ emission (with an eco-efficiency of 165.85%). Overall, although the composting process is an effective strategy for better use of resources, it will sometimes impose additional burdens on the environment if the external expenditures such as energy consumption and greenhouse gas emissions are neglected or less-intentioned.