A preparation process of PMMA／graphite nanocomposites was developed via in situ polymerization of monomers in the presence of sonicated expanded graphite with the aid of sonication．Expanded graphite was sonicated in a 70％ aqueous alcohol solution to get graphite platelets (NanoG) with thickness of 30～80 nm and diameter of 0.5～20 μm．The NanoG were dispersed in MMA monomers with BPO initiator．In situ polymerization was taken place when the mixture was heated under sonication．The TEM studies showed that the graphite nano-platelets were well dispersed in the PMMA matrix．Solution cast films of the nanocomposite were used for electrical conductivity measurements．The percolation threshold of polymer/NanoG nanocomposite films at room temperature was lower than 1.0 wt％

which was much lower than that of the composites with conventional graphite filler which was 5.5 wt％．The electrical conductivity reached 10-3 S／cm at a graphite content of about 4.0 wt％．The special structure of graphite nano- platelets with high aspect ratio of as high as 100～500 (width-to-thickness) played an important role in forming a conducting network within the PMMA matrix．