A structured, iterative methodology for user-centered design and evaluation of a Virtual Environment (VE) is newly proposed which includes not only an analysis of cooperative work by users within VE but also the whole designing method of user interface of the desktop VE system. A prototype training system has been constructed on the basis of the proposed method for teaching workers about the maintenance procedure how to calibrate the reactor coolant pump which is performed by several persons located in three separate rooms in the nuclear power plant; main control room, instrumentation rack room and pump room. Effectiveness of the proposed method has been confirmed by a laboratory experiment with scenario-based training course.