An optical klystron confi uration at millimeter wavelengths has been investigated to improve gain and efficiency of the free electron laser. A computer simulation code has been developed, and it can be applied to design an optical klystron device. For a small period wiggler case with a sheet electron beam at 300 keV and A and a wiggler period of 3.7mm operating at 300 GHz, 27.9 dB gain and 0.5% efficiency have been calculated. With a beam at 450keV and a wiggler period of 1cm operating at 160 GHz, 32 dB gain and 2.7% effecincy hive bsen cajoulabed.