The Changning-Menglian Belt in western Yunnan, China has been interpreted as a remnant of the Paleo-Tethys. The Nanduan Formation, exposed in the eastern part of this belt, is a prominent siliciclastic succession with a thickness of more than 2000 m. The age of this formation has remained poorly constrained due to the paucity of fossils. Ammonoids have been previously reported from this formation, but were neither systematically described nor properly illustrated. This study provides descriptions of ammonoids from two beds (Bed-2 and Bed-14) of the Banjiao section in the upper part of the Nanduan Formation. Specimens from Bed-2 preserve diagnostic suture patterns, facilitating generic determination, and thus five taxa are recognized, including Syngastrioceras sp., Somoholites sp., Epicanites sp., Stenopronorites sp., and Prolecanites? sp. Associated indeterminate small conchs from Bed-2 probably represent juveniles. Known ranges of these genera, and in particular, their co-occurrence, indicate a late Mississippian, and most probably a Serpukhovian age for the upper part of the Nanduan Formation. Only a single taxon, Fayettevillea sp., has been recognized from Bed-14, which also supports the Serpukhovian age.