The influence of a second harmonic frequency termination on fundamental output power has been experimentally investigated for Gunn diodes generating simultaneous oscillations. It is found that the fundamental power can be enhanced by a factor of 2 to 3 throughout the whole range of delayed domain mode by controlling the load parameter at the second harmonic frequency. The experimental observations also suggest that the oscillation at any frequency may be suppressed by the proper positioning of tho Gunn diode in the co-axial resonator. A tentative theory to explain the increase of fundamental power over a wide range of frequencies is also presented, taking into consideration the fundamental and the harmonic circuit parameters for Gunn devices operated in a resonator. © 1974 Taylor and Francis Group, Ltd.