Bridge is the hub of a road, playing an important role in transportation. With the age of bridges increasing in recent years, the condition of bridges is deteriorating. Bridge deterioration usually results from multiple factors. Studies in exploring the factors leading to bridge deterioration and its state estimation are booming. However, studies in the relationship between bridge deterioration and external environment by using database are still limited. Since 1992, National Bridge Inventory (NBI) has collected basic and historical information of bridges in every state, a very rich database which should include some valuable knowledge. Hence, through clustering and classifying in data mining, this study explores association rules for bridge deterioration, which is displayed in decision tree with clear routes for reference. Using association rules developed by thus study, bridge maintenance personnel can understand the future state of the bridge under their jurisdiction, respond to risks and costs of bridge deterioration, extend the service life of bridges, and protect the safety of road users.