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 dxy and dx2-y2 (dxz and dyz) interaction in Ir (Os) adsorbed structure.