This paper discusses the lifecycle simulation of mechanical parts that are managed by part agents to promote their effective reuse.It is essential to promote the reuse of mechanical parts with lifecycle management for the realization of a sustainable society. However, it is difficult to manage products in the use phase of product lifecycles owing to the unpredictable and uncontrollable behavior of consumers. This paper proposes a product lifecycle management system, called part agent system, using network agents and RFID tags attached on parts to promote reuse.A part agent generates advice for the user regarding the maintenance of corresponding parts based on its current state and lifecycle information. The lifecycle simulation scheme performed by part agents is described herein. First, a part agent expands the part lifecycle with time. Next, it estimates by simulating the deterioration of the part and subsequently selects preferable maintenance actions. To simulate forthcoming states of the part, the possibility of events related to the part is estimated based on the causal relation of events with the acquired data on the states of the part, user operations, and detected events. Herein, a proposed method is presented to describe how the deterioration process is represented as causal relations with probabilities and how the relation is created using simulation. The simulation method is described for two example cases: fatigue of spring and deterioration of joints of a robotic manipulator.