The Lincang batholith located in southern Three River region is the largest felsic intrusion in southwestern China. The Mengku pluton is one of the major alkali feldspar granite intrusions within the Lincang batholith. Based on field investigations, the authors studied the geochronology, petrology and geochemistry of the Mengku pluton. The results show that the pluton was emplaced in the Middle Triassic (236.2±3.7 Ma), much older than the previously conceived Early Cenozoic. This means that the distribution and abundance of magmatism of the Cenozoic intrusion province in the southern Three River region should be reevaluated. After a careful examination of whole-rock major and trace elements, Sr isotope and zircon trace elements, the authors believe that the Mengku pluton was of S-type affinity and originated from high extent of fractional crystallization of melts generated by metasedimentary crustal anatexis. The Mengku pluton is part of the Lincang batholith. The alkali feldspar granite is another important rock type that has never been reported in the Lincang batholith, which was previously considered to be composed of biotite monzogranite and granodiorite.