A regenerative gas turbine engine, with isothermal heat addition, working under the frame of a Brayton cycle has been analyzed. With the purpose of having a more efficient small-sized gas turbine engine, the optimization has been carried out numerically using the maximum power (MP) and maximum power density (MPD) method. The effects of internal irreversibilities have been considered in terms of the isentropic efficiencies of the turbine and compressor and of the regenerator efficiency. The results summarized by figures show that the regenerative gas turbine engine, with isothermal heat addition, designed according to the maximum power density condition gives the best performance and exhibits highest cycle efficiencies. (C) 2001 Elsevier Science Ltd. All rights reserved.