This paper deals with a learning environment for maintenance of plants and equipments based on virtual reality technology. First of all, maintenance tasks for operators are analyzed and the results of cognitive psychology are discussed from the viewpoint of understanding support system. Then design philosophy of the learning environment is proposed based on the analysis. The prototype system is designed and implemented using both EWS (Engineering WorkStation) and GWS (Graphic WorkStation). This prototype system is applied to the maintenance of the Gas Insulated Substation, which is one of power equipments, and its performance is evaluated by subjective assessment.