Abstract: The diagnoses of Dianomys, D. obscuratus and D. qujingensis are revised based on new material. The tooth originally described as P4 of D. obscuratus is actually dP4, Ml/2 and ml/2 are Ml and m2 respectively, and the p4 refened to D. obscuratus is transferred to D. qujingensis. D. qujingensis differs from D. obscuratus not only by larger size, but also in having forked metaloph on upper cheek teeth, mesolophid on ml, and more developed accessory crests in the mesosinusid on p4. A new genus, Youngomys, is erected to include two new species (Y yunnanensis and Y. pisinnus) and Youngomys? sp. The main characters of Youngomys include: cheek teeth being long and low crowned, having obtuse main cusps and slender lophs; upper molar being longer than wide, having metaloph oblique to protocone and low entoloph; on lower molar rather posteriorly extended posterior arm of protoconid lacking lingual part, and arm of entoconid weak. Y. pisinnus differs from Y. yunnanensis in smaller size, cheek teeth being lower crowned and more bunodont, upper molar having more lingually positioned hypocone relative to protocone, distinct metaconule and metaloph separated from protocone. The phylogenetic relationships of the Paleogene Ctenodactyloidea are analyzed based on the dental features using PAUP 3.1. l. It seems that the evolutionary process of the molariforrn or non-molariform of the fourth premolars is complex and may have occurred parallel in different lineages. Youngomys has close relationship with Mergenomys. Dianomys is a ctenodactyloid and may be a sister group to Mergenomys + Youngomys group plus the Ctenodactylidae.