Rb-Sr isochronal ages and chemical data are reported for the Alxa alkaline-rich intrusive rocks, western Inner Mongolia, China. These alkaline-rich intrusive rocks have elevated contents of total alkali w
O+Na2O) with an average of 12.70%. They are consisting mainly of K-feldspar, quartz, plagioclase, biotite and/or amphibole. Rb-Sr whole rock-minerals ages determination indicates that the isochronal ages are(250±18) Ma and (213.0±9.8) Ma for Suhaitu and Wenduerhao intrusion, respectively. The dated isotopic ages demonstrate that the Alxa alkaline-rich intrusive rocks belong to Indosinian alkaline magma activity. The similarities and differences of the alkaline-rich intrusive rocks distributed in the Alxa Fault Block and those distributed along the northern margin of the North China Block and the Tarim Block are compared and discussed. The authors deduce that during late Palaeozoic-Indosinian period, the Alxa Fault Block and the North China Block may be located in the same geodynamic system, but the Tarim Block may be located in a different geodynamic system.