The Freedericksz transition of homeotropical alignment liquid crystal doped with little FeTPPCl, which was induced by a perpendicular de magnetic field, has been studied theoretically and experimentally in this paper. And the doped 5CB was observed to have dramatically decreased the critical magnetic field for bend reorientation, only one fourth of that needed for pure 5CB. This phenomenon is due to the intense paramagnetic of FeTPPCl [5,10,15,20-Tetraphenylporphineiron(III) chloride] and the interaction between the host and guest in the form of the coordination of -CN in 5CB onto the metal ion of the porphyrino. However, 5CB doped with ZnTPP [5,10,15,20-Terraphenylporphinezinc(II)] exhibited no decrease in the critical magnetic field. This result is very useful in liquid crystal display as magneto-optic component.