The perfect absorption in monolayer graphene at terahertz frequencies is investigated. The designed absorber consists of a monolayer graphene separated from a dielectric multilayer structure by a dielectric film. Nearly 100% absorption is observed at resonance, which results from to the excitation of graphene Tamm plasmon polaritons at the boundary between the graphene and dielectric multilayer. The distribution of the electric field intensity along the z-axis is also presented to disclose the physical origin of such perfect absorption phenomenon. Besides, the operating frequencies can be flexibly tuned through changing the angle of incidence, which is particularly attractive as the graphene absorption is not sensitive to the polarization state. Furthermore, the absorption properties can be maintained with large fabrication tolerance, which is advantage for practical fabrication. Lastly, it is found that the operating frequency can be easily tuned by a change in the gate-voltage. It is believed the conclusions provide the potential for combining graphene with metamaterial to enable novel THz tunable device applications.