Abstract：The major granitoid rocks in a Late Paleozoic rift area, southeastern Inner Mongolia, are quartz diorite, tonalite, granodiorite, monzogranite and syenogranite. SHRIMP zircon U-Pb dating of the quartz diorite yielded ages of 313±5-323±4 Ma, indicating a Late Carboniferous age. Both the Qianjinchang and Daqên monzogranite plutons are intruded into paralic clastic rocks of the Permian Shoushangou Formation (P1ss) with a clear intrusive contact relationship. The SHRIMP U-Pb zircon ages for the two plutons are 280.8±3.6 Ma and 281.5±0.5 Ma respectively, indicating a late Early Permian age. The Ulan Tolgoi syenogranite has an age of 259 Ma and the Yangjiaolin Gol monzogranite has U-Pb ages of 246-216 Ma, suggesting that another intrusion peak occurred during the Late Permian-Triassic. There were three intrusion peaks in the Carboniferous-Permian (partly Triassic) rifting period, i.e. the Late Carboniferous quartz diorite intrusion peak, Early Permian tonalite-monzogranite intrusion peak and Late Permian monzogranite-syenogranite intrusion peak.