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Mechanisms of drag reduction due to flow control around a circular disk using a coaxial type dielectric barrier discharge plasma actuator (DBD-PA) are numerically investigated at Reynolds numbers (Re) of 5000 and 10 000. The disk is placed normal to the freestream direction and the coaxial type DBD-PA is installed on the upstream side of the disk surface. Flow induced by the coaxial type DBD-PA in quiescent air and a time-averaged drag coefficient of the case without the control are in favorable agreement with experimental results. It is found that two phenomena on flow dynamics are responsible for the drag reduction due to the flow control of the coaxial type DBD-PA. First one is the generation of low-pressure region because of an interaction between the induced flow and the freestream on the upstream side. Second is the delayed roll-up motion of the separated shear layer from the edge of the disk. Most of cases demonstrate the reduction of the time-averaged drag and the reduction rate is up to 2.3% at the Re = 5000 and 3.0% at the Re = 10 000, respectively. Moreover, the control effects on surface pressure distribution at both Reynolds numbers present trade-off and the large relative diameter of the coaxial type DBD-PA is the favorable impacts in terms of the time-averaged drag coefficient and the drag fluctuation. [ABSTRACT FROM AUTHOR]