AbstractCharge transfer in perovskite oxide heterostructures could break the delicate balance among charge, spin, orbital and lattice order at the interface, producing exotic phenomena that cannot be observed in bulk materials. Here, opposite interfacial charge transfer directions are observed in SrIrO₃/NdNiO₃ and SrIrO₃/LaNiO₃ 3d/5d perovskite heterostructures. This is accompanied with an inverse change of Ni e_g orbital polarization and Ni-O pd hybridization across the interface, by stretching/compressing the out-of-plane Ni-O bond in the opposite internal electrical field due to the opposite electron transfer direction. These interfacial reconstructions finally bring about a manipulation on the transport and magnetic characteristics. This work reveals that A site cation in perovskite heterostructures could be a knob to control the interfacial charge transfer direction, and the 3d/5d perovskite interfaces are excellent platform to study the complex interplay between various order parameters and stimulate novel interfacial effects.