Geochronology, petrology and geochemistry were performed systematically on basic dykes in Mianning, Sichuan province. On the basis of major and trace elements patterns, basic dykes belong to tholeiitic basalt serie in lithology and have OIB (Ocean island basalts) features, which were homologous with Emeishan basalts. Trace elements tracing and Th/Hf—Ta/Hf identification diagram show that basic dykes were derived from mantle plume with little contamination. The LA ICP MS U Pb zircon data of basic dyke yields a concordant age, 256.7 4.3Ma, which may represents the rock mass crystallization time. It shows that the Mianming basic dykes were formed in the late period of Emeishan mantle plume magmatism. In addition, the dating implies that dykes experienced geological events occurred about 244Ma, which was caused by thermal erosion of magma and triggered by the Indosinian movement.