Abstract: Sumatra is located in the most west of Indonesia and is subject to the subduction of the India-Australia Plate beneath the Eurasian Plate.Sibolga, one of the largest granites in Sumatra, still lacks high-precision geochronological research, which prevents the advance of knowledge in its petrogenesis and tectonic evolution.Biotite monzogranite from Sarudik area, Sumatra was assigned to granite of A1 type by previous geochemical research.The present study conducted LA-ICP-MS Zircon U-Pb age analysis on the identical granite, yielding an age of 225.3±0.8 Ma and indicating its formation in late Triassic.Combined with the results from previous study, it is proposed that Sibolga and adjacent areas were dominated by within-plate tectonic environment during late Triassic period, which coincides with the results from regionally geological studies.It is further concluded that late Triassic A1-type granite in study area is associated with locally crust extending and thinning throughout Sumatra during mid-late Triassic.