The crystal structure and its evolution of an n-type La20.4FeCo3 Sb12 compound have been characterized by the Rietveld refinemen t and TEM observation. An interstitial site 01980nm in radius has been found in the lattice of t he binary Skutterudite compound. Filling this interstitial site by rare earth at om mainly results in a change of lattice coordinates of the Sb atom in the compo und. Thermal vibration parameter of the rare earth atom is larger than those of other atoms. The electron diffraction patterns confirm the bcc structure of this compound. The intensities of diffraction from different crystallographic planes correspond well between electron and powder x-ray diffractions.