Tunable graphene plasmonic whispering gallery mode nanodisks formed by nanoscale chemical potential variation on an infinite freestanding graphene monolayer at near-infrared wavelength range are proposed and numerically investigated in this paper. The Q-factor of a high-order whispering gallery mode of 136 and an extremely small mode volume of 4.8×10−7 (λ0/n)3 with a large Purcell factor of 1.7×108 is achieved in a cavity radius of 10nm. This graphene based nanodisk cavity structure with an ultrasmall mode volume and large Purcell factor is a promising candidate for high efficiency plasmonic emitting sources, filters, or a key component in the fields of transformation plasmonics and plasmonic integrated circuits technique in the future.