Abstract: Heterostructures composed of perovskite oxides with different properties provide an opportunity to observe emergent phenomena. A key feature of oxide heterostructures is interfacial electronic and orbital reconstruction. In this talk, we will demonstrate a dramatic variation in magnetic anisotropy caused by electric tuning of the charge-transfer process in a La_0.8Sr_0.2CoO₃/La_0.67Sr_0.33MnO₃ bilayer structure. By repeatedly changing the valence state of Co ions in the La_0.8Sr_0.2CoO₃ top layer using ionic-liquid gating, reversible switching of the magnetic easy axis of the bottom La_0.67Sr_0.33MnO₃layer between the out-of-plane and the in-plane direction is achieved, accompanying a modulation of the interfacial exchange coupling. Mn-to-Co charge transfer and its effects on the interfacial orbital occupancy are further confirmed by x-ray absorption spectroscopy and x-ray linear-dichroism analysis. The considerable interfacial charge transfer causes an overlap of the Mn and Co 3d orbitals, resulting in orbital reconstruction in the La_0.67Sr_0.33MnO₃ layer and thus magnetic anisotropy. This work demonstrates a promising method for tuning the orbital occupancy and related properties of perovskite heterostructures.