Abstract: The high sensitivity strain gauges were laid on the U-rib butt welds of fast lane and slow lane in a quarter-span of a bridge to measure the strain time curves. The curves were filterred and extremum were wiped off to obtain the real strain time curves of steel box girder under the vehicle loading. The corresponding fatigue stress spectrums were drawn with the rain flow method. Fatigue damage degree of U-rib butt welds under each stress ampiltude and cycle was analyzed, and its fating lifes of fast and slow lane were obtained, combining with British BS5400 specification. The result shows that the fatigue service life of U-rib butt weld of slow lane is smaller than that of fast lane, the latter is 1.5 times of the former. In addition, compared with the position of both sides of U-rib weld bottom, the position of center of U-rib weld bottom is more serious and need more attention.