Public sewage treatment plants in Japan consume 6.3 billion kWh of electricity and produce 350 million tons of sludge annually, with serious impacts on the local and global environment. In this study, a Life Cycle Inventory of a sewage treatment plant for domestic wastewater was done to assess the environmental effects using data from statistics and previous studies. The system boundaries include sewage pipe construction and operation, sewage treatment plant construction and operation, material production and sludge treatment. The functional unit is defined as the treatment system of wastewater produced by one person in one year. Energy consumption and global warming are studied as the environmental parameters. The following conclusions have been drawn from the results. 1) Energy consumption and emission of Greenhouse Gases (GHGs) increase exponentially as the population density decreases. 2) If the actual population is less than the planned population, the operation efficiency of the sewage treatment plant decreases. 3) The energy consumption per person per year is 1,026 MJ, and CO2 equivalent emission factor is 89 kg. 4) Energy consumption for sewage treatment plant operation accounts for 78% of the total energy consumption. Under the assumption that 20% of the total carbon in the sludge is converted to CH4, the CH4 emitted from sludge landfill and the N2O generated by sludge incineration contribute respectively 35% and 14% to global warming. These results also show that further development of sludge recycling technology is desirable to decrease GHGs.