The isothermal crystallization behavior of nano-alumina particle-filled poly(ether ether ketone) (PEEK) composites has been investigated using differential scanning calorimeter. The results show that all the neat PEEK and nano-alumina-filled PEEK composites exhibit the double-melting behavior under isothermal crystallization. The peak crystallization times (tp) for all the neat PEEK and PEEK/aluminum oxide (Al2O3) composites increase with increasing crystallization temperature. Moreover, the crystallinity of the PEEK/Al2O3 composite with 7.5 wt % nano-filler content reached the maximum value of 44.8% at 290 degrees C, higher than that of the neat PEEK polymer. From the lower value in tp and higher value in Xc for the PEEK/Al2O3 composites, the inclusion of the nano-alumina into the PEEK matrix favored the occurrence of heterogeneous nucleation. The Avrami exponents n of all the neat PEEK and PEEK/Al2O3 composites ranged from 2 to 3, and the n values for PEEK/Al2O3 composites were slightly higher than that of the neat PEEK polymer, indicating that the inclusion of the nano-filler made the crystallization mechanism more complex. However, the growth rate of crystallization was lowered as the nano- filler was introduced, and the decrease in growth rate reduced the grain size of the PEEK spherulites because of the lowering of molecule mobility during isothermal crystallization. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012