The optimum size of Micro Gas Turbine Cogeneration Systems (MGT-CGSs) in a Sewage Treatment Plant (STP) in terms of its economic performance was investigated. A STP operating in a cold region was adopted as a model and was scaled down to obtain different size ratios. It was also assumed to operate in different regions to obtain different heat demand patterns. MGT-CGSs with power output capacity of 30, 65 and 200 kW were simulated to utilize biogas produced by the STP. Instead of multiple units of the same size of MGT-CGSs, combination of different sizes of MGT-CGSs was also investigated. Life Cycle Cost Analysis was carried out to compare the economic performance of MGT-CGSs. It was found that optimum combination of three types of MGTs (MGT-Combined) stated above had the highest power generated and efficiency. However, MGT-Combined also had larger power generation capacity and low usage ratio, thus resulting in higher capital investment. Although all configurations of MGT-CGSs can generate Net Present Value, optimum configuration was obtained when the rated fuel input of MGT-CGS is approximately equal to the biogas production of the STP. However, when heat demand fluctuates throughout the year smaller size of MGT is preferred. (C) 2017 Elsevier Ltd. All rights reserved.