The results of Rb-Sr isotopic dating of granitoids from the S«ng MiÖn Complex are presented as follows: 6 whole rock samples (coordinates: 22⁰58^' N; 105⁰03^' E) have been giving the value of 173 ± 14 Ma., with initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.767 ± 0.010; 5 other whole rock samples (coordinates: 22⁰54^' N; 105⁰06^' E) have been giving the value of 172 ± 16 Ma., with the initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.739 ± 0.015. Combined with research results on geological and petro-geochemical characteristics of these rocks, the above said new data have been proving the Jurassic age of S«ng MiÖn Complex, and its crustal origin corresponds to geological materials. The TSN 206 SA mass spectrometer has been used in the analysis of isotopic ratio of Rb and Sr.