Poly(arylene ether nitrile) (PEN) nanocomposites filled with functionalized graphite nanoplates (GNs) were prepared by a simple solution- casting method and then characterized by rheometer and thermogravimetric analysis (TGA). This study investigates how the surface treatment of GNs affects the GN dispersion state. The linear rheological test indicated that the 4-aminophenoxyphthalonitrile-grafted GN (GN-CN) presented better dispersion in PEN matrix than purified GN because the corresponding composite showed the lower rheological percolation threshold, which was further confirmed by scanning electron microscopy and solution experiments. The TGA revealed that the presence of 4-aminophenoxyphthalonitrile-grafted GN retarded the depolymerization evidently compared with that of purified GN, showing remarkable increase in the temperatures corresponding to a weight loss of 5 wt % (increased by 21 degrees C) and maximum rate of decomposition (increased by 9 degrees C). Both the dispersion state and the surface functionalization of GN are very important to the thermal stability of PEN matrix. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 127: 1827-1833, 2013