The Hoover Dam Bridge is the 578m long and 26.8m wider bridge with 4 carriage lanes and on pedestrian way. The bridge has the twin ribbed arch bridge with the span of 323m over the Colorado River just below the historic Hoover Dam completed in 1936. The bridge will be the longest concrete arch in the North America and the fourth longest in the world. The arch segments were individually constructed via cast-in-place concrete using temporary stays and pylons. The twin ribs are connected by eight steel struts at the spandrels. The bridge will be completed in summer 2010. This paper mainly presents construction of the concrete arch portion of the bridge.