Large magnetocrystalline anisotropy energy (MAE) is a critical requirement for nanomagnets for applications in magnetic memory and storage devices. Due to small spin–orbit interaction the MAE of ferromagnetic films or single molecule magnets based on 3d metals is small and in typical magnetic nanostructures it is of the order of 2–3 meV. We show that MAE as high as 140 meV can be achieved by applying an external electric field or a biaxial tensile strain to phthalocyanine sheets decorated by 5d transition metal atoms such as Os and Ir. Our observation is based on a systematic study of 5d transition metal absorbed ploy phthalocyanine (Pc) sheets using first-principles density functional theory (DFT) combined with self consistently determined Hubbard U that accounts for the strong correlation energy. We attribute the high MAE values to d_xy and d_x²–y² (d_xz and d_yz) interaction in Ir (Os) adsorbed structure.