Due to the large spin orbit interaction effects, the magnetic properties of rare earth ions are of vast complexity to understand. This presentation introduces the investigation on magnetic anisotropy of heavy rare earth ions. Two sorts of heavy rare earth ion containing complexes are synthesized and the magnetic principal axes are determined by the angular resolved magnetometer measurement on single crystal. The orientation of the quantized axis is analyzed by placing the aspherical 4f-shell in the electrostatic potential field.