A VR (Virtual Reality)-based cooperative training system is proposed for machine maintenance in nuclear power plant. The NETCOM (NEtworked-vr based Training system for CoOperative machine Maintenance) system has been developed on basic software called LASNET (Large Scale Networked virtual EnvironmenT), which can offer flexible CSCW environment for networked virtual environment with multimedia communication between participants. By using the NETCOM, instructor can teach a group of trainees a maintenance procedure how to calibrate the indication of a reactor coolant pump in separate rooms. It was verified by an experiment that the NETCOM system would give a sensation of presence in the course of participated training.