We will demonstrate the feasibility of achieving giant magnetic anisotropy energy (MAE), an important spin-orbit coupling induced phenomenon, in systems with transition metal dimers on defected and decorated graphene.In particular, either a Pt-Ir dimer on a single vacancy or an Os-Ru dimer on a nitrogen-decorated divacancy possesses an MAE larger than 60 meV and high structural stability.Interestingly, their magnetic anisotropy can be conveniently manipulated by using external electric field.These features make them good candidates for the use in room temperature spintronics and quantum computing devices.