A study on nonisothermal crystallization kinetics in fly ash (FA) filled isotactic-polypropylene (PP) composites has revealed some interesting phenomena. Composites made by injection moulding of PP with 0, 20, 45, and 60 wt % of FA were nonisothermally studied using differential scanning calorimetry at cooling rates 10 degrees C, 15 degrees C, and 20 degrees C per min from a melt temperature of 200 degrees C cooled to -30 degrees C. Whilst neat PP showed a mono modal alpha crystalline phase- only structure, presence of FA led to bimodal thermographs revealing partial transcrystallisation of alpha into beta, to maximum 14%. The onset and peak crystallization temperatures of all samples decreased by similar to 3 degrees C with each 5 degrees C/min increase in cooling rate. Parameters such as crystal growth rate, dimensions, and activation energy were determined using a series of established models. The Avrami graphs showed that contrary to the published data, there are two sets of straight lines (a) with a lower slope at low cooling rate and (b) with a distinctly higher slope for high cooling rate. Activation energy of the materials reached a maximum at 45% FA. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1510-1517, 2010