This paper describes the LCA environmental impacts resulting from fuel fired co-generation systems using our NETS (Numerical Eco-load Total Standard-[1]) scheme having GUI (Graphic User Interface). Following a brief introduction of the NETS evaluation method, firstly the total eco-load from commercial electricity power plants in Japan is numerically calculated in the NETS unit. We take here almost all factors into account including fuel mining, transportation and processing, plant manufacturing, assembling and its building construction, fuel consumption and maintenance during twenty durable years, plant dismantling and waste processing, thermal efficiency and delivery loss, etc. This evaluation greatly helps in the eco-load estimation of co-generation plants when electricity purchase is needed during the plant operation with a minimum eco-load. Next, co-generation systems are paid attention to their ecological effiectiveness. Here, we select different fuel fired co-generation plants of natural gas, heavy oil, and coal. Environmental impacts resulting from the respective plants include fossil fuel depletion and natural resources depletion, global warming due to green house gases, ozone layer depletion due to CFC type gas emission, various water and air pollution, rain acidification due to NOx and SOx, waste processing, recycling etc. Each item can be successfully calculated from the inventory database accumulated by authors. Hourly energy demands of electricity and heat (steam, hot water and cold water) are set to a typical model for eight office buildings and two hotels. The NETS method is applied to the co-generation model, resulting in the best cost-operating scheme of co-generation plants with a minimum environmental impact.