This bridge which is known as the old bridge, is situated in the south of Zanjan and on Zanjanroud (Zanjan River). The length of the bridge is approximately 100 m, its width is 6.70 m and its height in the highest point is 12 m from the bottom of the river. The old bridge has 3 arches for the water to pass and also four decorative, arches, and it was built in Naser-od-din Shah Qajar era.
The century-old Mir Baha-ol-Din Bridge or Old Bridge is the oldest and largest standing bridge on the Zanjan River (Zanjan Roud) which was built in the Qajar era (1785-1925) by a merchant of the same name. This brick bridge is 100 meters long and 7 meters wide. The highest point of the bridge is 11 meters from the water. The bridge has a large middle arch with two smaller arches on either side. This bridge was registered as a National Heritage Site in 1977.