Abstract.A E-band folded waveguide traveling-wave tube (FWTWT) is modelled, and the nonlinear interaction between the electron beam and electromagnetic field is investigated by utilizing a 3-D particle-in-cell (PIC) code, MAGIC3D. The process of the interaction for millimeter wave TWT are presented, including the longitudinal momentum of the electron, the averaged radiation power, and the influence of input power on output power. The radiation power and gain of the FWTWT at 85GHz are 88.5W and 27.5dB.The output power of the FWTWT changes only 1 dB across the 6-GHz bandwidth ranging from 82GHz to 88GHz, which is attractive for many applications, e.g. 5G communication networks , satellite communication, electronic counter measures.