A LCA (life cycle assessment) scheme for any industrial activity system is introduced to estimate the quantitative load on the environment with the aid of the NETS (numerical environment total standard) method proposed by the authors as a numerical measure. Two kinds of environmental loads respecting fossil fuel depletion as input resources to the system and global warming due to CO2 emission as output are taken into account in the present eco-criterion, in which the total eco-load (EcL) value is calculated from the summation of respective environmental load factors on the whole process in a life cycle of the system. This NETS method is applied to eco-management co-generation systems, in which a computer-aided output navigator proceeds the LCA estimation with ICON and Q&A communication. An operation scheme most friendly to the environment with a minimum EcL value, i.e., an eco-operation scheme, is derived from the optimization theory.