Abstract: The Upper Ordovician Longmendong Formation on the southwestern margin of Ordos platform is disputed since its nomination. Its main difference from Pingliang Formation is the formation age. The former consists of five graptolite zones, which are Glyptograptus teretiusculus zone, Nemagraptusgracilis zone, Climacograptus peltifer zone, C. longxianensis zone and C. spiniferus zone, but the latter one only comprises the first two graptolite zones. Recently, through the study on this area, two more graptolite zones have been discovered in Pingliang Formation,including Climacograptus bicornis zone and C. longxianensis zone. As a result, the age range of Pingliang Formation is nearly same as the one of Longmendong Formation. In addition, they have the same lithological characters and are formed in the same tectonic environment, so they are synonym.According to the stratum nomination rule, the Longmendong Formation is suggested to be abandoned, while the Pingliang Formation is suggested to be used.