Abstract: This paper presents geochronological and whole-rock geochemical data of Niuduo biotite monzogranite exposed in the north of Lancang granitoids to discuss the origin of the Niuduo biotite monzogranite and its geochemical characteristics. Zircons of biotite monzogranite show no euhedral crystals and have clear oscillatory zones as well as high Th/U ratios, suggesting magmatic origin. Zircon LA-ICP-MS dating of the biotite monzogranite yielded an age of 243.6±1.4Ma(MSWD=1.00, n=15), implying that the crystallization of the biotite monzogranite body took place in Middle Triassic. Petrological and geochemical studies show that it is characterized by high silicon and aluminum as well as abundant alkali, with A/CNK being 1.05~1.08, thus belonging to high-K calc-alkaline series. The whole-rock geochemical characteristics such as strongly fractionated REE pattern, apparently negative Eu anomalies and Ce anomalies, depleted Nb, Ta, P, Zr, Ti, and high Rb, K, Th, U suggest that it belongs to S-type granite. The authors hold that the melting took place at about 25km in the middle crust due to extension of the thickened crust after the closure of the paleo-Lancang Ocean. The results obtained by the authors support the hypothesis that the Lancang River tectonic zone represents a remnant Paleo-Tethyan main ocean preserved in the collision suture along a Gondwana-derived microcontinent.