This paper focuses on the mechanical slip behavior of bolted box girder connections consisting of slip-critical flange and web joints. Flange and web splices of girder connections are usually designed separately. However, actual bolted girder connections resist the applied bending moment through the cooperation of flange and web resistances, which is called cooperative mechanisms. By evaluating a cooperative mechanism quantitatively, we can comprehend the safety margins for overall slip strength to the design force, and the required number of bolts of the connections can be decreased. In the present study, FE analysis of bolted box girder connections was conducted to evaluate the slip strength considering the cooperative mechanism. As a result, the initial slip strength, when the stiffness of the box girder bolted connection starts decreasing, is always larger than the flange design slip moment. It was confirmed that the bolted box girder connections had cooperative resistance mechanisms. In all cases, the overall slip strength was larger than the summation of the design slip resistance of the flange and rib splices and 80% of the web splice.