Total energy system of gas turbine is a gas turbine powered energy plant which generates the heat output besides the shaft power. In this paper, the basic thermodynamic characteristics of the plant which satisfies two requirements, such as electric power generation and central heating and cooling, are discussed. The high temperature of turbine exhaust causes a lower thermal efficiency than other conventional prime movers, but by recovering the wasted heat by boiler from 60% to 85% of the input energy is converted to shaft power and available heat. And the excess air ratio in gas turbine cycle is so large that supplementary fuel can be fired in the waste heat boiler. Thus, by controlling the supplementary fuel and the by-pass valve of the regenerator, this system shows fine flexibility in its operation. Also, the characteristic chart of this total energy system in part-load is presented for determining the operating conditions.