Abstract: Simulations of millimeter-wave free electron laser amplifiers with a typicat rectangwiar wars were made using a 3-D free electron laser amplifier code (WAGFEL).Te results were in good agreement water those of linear theory.Meanwhile,effects of the beam current.energy spread and space charge on gam coefiacnt-were examined.Furthermore,the dependence of values of saturated outputs on the electron current.in position of saturation was studied and some useful relations for scaling were obtained.