Abstract: This paper makes a detailed study on the chronology and geochemistry of biotite monzonitic granite from Hongmiaogou area in the northwest margin of the South Qilian. Zircon U-Pb dating of biotite monzonitic granite yields a weighted average age of (479.7±3.4)Ma (MSWD=0.047), which indicates this granite had emplaced in Early Ordovician. Geochemical data shows that this pluton has geochemical characteristics of I-type granite and belongs to peraluminous series (A/CNK=0.9~1.04). It has relatively low total REE contents (∑REE=67.55×10^-6~83.07×10^-6) with LREE enrichment and Eu negative anomaly. It shows the geochemical characteristics of arc magma, with an enrichment of some large ion lithophilc elements (Rb, Ba, K) and loss of Sr and high field strength elements (Nb, Ti, P). Comprehensive research presents that Early Ordovician biotite monzonitic granite was the product of arc magma and caused by the subduction of South Qilian Ocean. It records the significant event of plate tectonics in early Ordovician and provides useful information, important basis to understand the tectonic evolution of South Qilian orogenic belt.