The study aims at improving the adsorption performance of montmorillonite for relatively large molecules. Making use of the property that phenanthroline can form coordination compounds by reacting with the transitional metal cations, the authors modified montmorillonite by using two different methods. The effect of the formation forms of coordination compounds on the structure and the absorption performance of modified montmorillonite were studied by measuring its decolorization performance for dye and analyzing its X-ray diffraction. In this paper, the thermal stability of modified montmorillonite was investigated too. The results show that the interlayer spacing of montmorillonite can be further enhanced through the reaction of montmorillonite containing transitional metal cations with phenanthroline to reach over 1.76nm. It is shown that the two-stepsynthetic method, by which metal cations are introduced into the interlayer of montmorillonite and then the coordination compounds are in-situ synthesized, hasa better effect. In addition, modified montmorillonite shows a fairly good thermal stability up to 400℃.