Novel CA modified starch–CMC–MMT bionanocomposite films were prepared and the effect of MMT on the structural and physical properties of the starch–CMC biocomposites was studied. An increment in the interlayer distance of the clay sheets in the bionanocomposites was observed by X-ray diffraction providing evidence that the clay nanolayer formed an intercalated structure without reach a complete exfoliation. But there was no any sharp d0 0 1 peak in XRD pattern of pure starch–MMT nanocomposites. This result confirms the extensive diffusion of polymer chains inside galleries of clay. At the level of 7% MMT, bionanocomposite films showed the lowest solubility (7.21%). By addition of 3 and 7% MMT, the moisture absorption of the films decreased from 23.46% to 20.00 and 14.78%, respectively. Furthermore, MMT also demonstrated an important influence on the mechanical proper- ties. Increasing MMT content from 0 to 7% (w/w), could increase UTS from 9.83 to 27.55MPa. However, the SB decreased signifi- cantly (p≤0.05) from 63.52 to 18.25%. The temperature position of the melting peak increased from 221.5 to 243.0 ◦ C as MMT content increased from 1 to 7% (w/w) starch. By increasing of MMT contents, the glass transition disappeared gradually and no glass transition was observed in the thermograms of the films containing 5 and 7% MMT. These results indicate that intercalation of MMT layers within the starch chains occurs in amorphous regions.