The development of a VR based training system is presented for teaching disassembling procedures of mechanical machines used in nuclear power plant. The methods of Petri net model for describing trainees¡Ç plausible actions in the disassembling process and reducing a right sequence of action sequence are developed as well as realization of the related Petri net editor and the demonstration of the developed VR based training system was demonstrated by example practice of disassembly simulation of check valve. Moreover, the needed future works are also discussed.