Polylactic acid (PLA)/clay nanocomposites were prepared by secondary melt blending of modified nanometer clay and PLA. The TEM showed that the higher exfoliation could improve the dispersity of clay in PLA, thereby enhance the interfacial force between the two phases, and make the phase interface equipped with highly interfacial plasticization region. The DSC showed that the exfoliated clay played a role as nucleating agent in nanocomposites and increased the crystalline of the nanocomposites. The mechanical properties of nanocomposites were improved owing to the above two reasons. When the mass fraction of clay was 2%, the exfoliation degree of clay in PLA/I.34TCN nanocomposites reached the highest, and the elongation at break of nanocomposites was significantly increased by up to 101.00%, which was 18 times that of pure PLA. The TG showed that the thermal stability of the nanocomposites increased with the increasing in the content of I.34TCN.