Biochar is a kind of highly aromatized substance with very stable physical, biological and chemical properties, and hence deemed as a measure of great potential for carbon sequestration. However, once biochar is applied into soil, it may expedite mineralization of native soil organic C, which is called “priming effect”. In this study, biochars was produced out of C4 plant, Miscanthus giganteus at 350℃ (BC350) and 700℃(BC700), and then amended separately into two soils, 3.70 and 7.60 in pH that had long been cultivated with C3 plants for in-lab incubation at 25℃ for 180 days. The respiration of CO₂ and δ¹³C were measured at set intervals during the incubation to verify andquantify priming effects of the biochars. Results show that in the soils amended amended with biochar, BC350, in particular, mineralization of organic carbon was obviously higher, that is to say, significant positive priming effect was observed. The priming effect in the acid and alkaline soils was CO2-C 250 and 319 mg kg^-1 soil, respectively. Besides, mechanisms of the priming effects biochar are also discussed from the angle of physics, chemistry and biology in this paper.