Organic montmorillonites (O-MMT) were prepared by treating Na-montmorillonites with alkylammonium ions．The polypropylene／montmorillonite nanocomposites (PMNC) were prepared in two steps．The mother materials were synthesized by an in-situ graft-intercalation method with maleic anhydride and styrene firstly

then PMNC were prepared by blending PP and the mother materials by using a twin screw extruder．XRD results suggested that the layer spacing (d001) of the organic montmorillonites increased from about 1.49 nm up to about 2.96 nm

and the layer spacing (d001) of montmorillonitenm in PMNC from about 2.96 nm up to about 4.0 nm．Mechanical performance tests showed that the noched impact strength (11～12 kJ／m2) and the tensile strength (38～39 MPa) of DMNC with lower montmorillonite contents(1wt％～3wt％) were higher than the noched impact strength (4.12 kJ／m2) and tensile strength (33 MPa) of PP．The mechanical performance of PMNC is superior to PP’s．The morphology of impact fracture surfaces were investigated by SEM and the mechanism of impact fracture was discussed．With the increasing of montmorillonite content

the mechanism of impact fracture was converted into toughness fractures from frangibility fractures．