Abstract: The numerical simulation is a very effective mean to the research and design of microwave devices. At present, Particle-In-Cell (PIC) is an usual numerical simulation. Tech-X Corporation develops PIC software with the name as VSim, which can supply the simulation of the particles in microwave and THz devices. VSim can be used in performance optimization and reliability analysis of these devices, and research of the complex media's electromagnetic property. Using VSim, the evolution of electromagnetic fields and plasma, phase diagrams can be recorded, which can give people deep understanding of the physical mechanism in these devices.