Millimeter waves have been extensively studied for many applications including high-resolution radars, communications, materials processing, plasma heating, high energy particle accelerators, and power transmission. The authors have developed a vacuum electronic device based on field emission mechanism for the generation of K-a-band millimeter waves by following the previous successful design of the terahertz devices. The design of the device has been carried out and verified by the numerical simulations. The cold tests and the hot tests have been done using the,simulation tools SUPERFISH and MAGIC, respectively. A pure resonant frequency about 35 GHz is obtained. The simulation results show that an electronic efficiency up to 9.16% can be achieved with no external magnetic fields. The success of the design for millimeter waves and terahertz waves implies that the interaction mechanism has general significance and should be further investigated. (c) 2007 American Vacuum Society.