First experimental results on microwave radiation at double plasma frequency from plasma heated by ~100 keV, 2-10 MW electron beam of ~100 s duration in GOL-3 are presented. Measurements were done in 75-200 GHz band. The spectrum was peaked at ~94 GHz at ~31013 cm-3 plasma density, the temporal behavior of radiated power was spiky. The generation efficiency reached ~1% of total beam power.