In the past decade, remanufacturing has been increasingly recognized as an important approach for realizing long-term efficiency and sustainability in manufacturing industry. A considerable number of tools and methods have been developed to facilitate the planning and operations of remanufacturing process. However, most of the studies focus on the end-of-life phase of product, which may lead to more negative environmental impacts, increased energy use and higher operational cost. Proactive remanufacturing is a new strategy that allows us to determine the optimal remanufacturing timing based on the health condition of a product. The research on environmental sustainability from proactive remanufacturing is still in the early stage. In this paper, we develop a stochastic program to proactively determine the remanufacturing timing with the objective of minimizing the total environmental impacts. Our preliminary results show that proactive remanufacturing strategy can further reduce the environmental impacts compared with the reactive or end-of-life remanufacturing scheme.