Recently, endurance fatigue life in a long span bridge has been lively studied together with investigation of superannuated bridges. Design load of bridge and the damage of deck plate have been examined from the report of the investigation of superannuated bridges. In this paper, it can be seen that the damage of Deck plate is caused by crack and abrasion because numerous passing vehicles and overweight vehicles pass through the bridge. In the design of a long span bridge, steel plates are much used to reduce self-weight, so that deck plate is finely cracked in such a heavy structure as steel bridges. Accordingly, the distribution of the volume of future estimated passing vehicles are obtained from the statistics of the volume of traffic passing through Seoul-Pusan expressway. The volume of the estimated passing vehicles is calculated by the number of equivalent axle load cycle, and endurance life is obtained by applying equivalent axle load cycle to fatigue propagation life of the steel deck plate. In this paper, the estimated endurance life is obtained by applying the estimated volume of passing vehicles to fatigue crack propagation life of steel deck plate. The major findings in this study can be summarized as follows: 1) The damage of deck plate in superannuated bridges is mostly caused by crack. Such damage is brought out by numerous passing vehicles and overweight vehicles. 2) Curves of axle load of the volume of traffic shows a pair of peaks which resembles two-mountain figures. This means that loaded vehicles pass together with empty vehicles in case of heavy vehicles. 3) Axle load vehicles under 6.9t do not affect the equivalent axle load cycle, which is good enough to ignore the estimation of endurance life of the steel deck plate in bridge. 4) Considering the number of future passing vehicles, the crack propagation life of steel deck plate, if the design life of a bridge is 100 years, is proved to have sufficient the endurance life. 5) The fatigue life of such a heavy steel structure as a long span bridge is evaluated by crack initiation life, failure life, and crack propagation life, but steel deck plate is strong enough for passing vehicle load cycle even if only fatigue propagation life is calculated.