In the present study the effect of different size of graphene nanoparticles 5,15 and 25 mu m in diameter, on melting behavior and crystallization kinetics of high density polyethylene (HDPE) was investigated. Crystallization of HDPE and its graphene nanocomposites in non-isothermal conditions was studied at several constant cooling rates from 1 to 20 K/min by means of differential scanning calorimetry. The results showed that the Avrami analysis' modified by Jeziorny and a method developed by Mo could describe the non-isothermal crystallization of nanocomposites very well. Kinetics data analysis showed that the overall crystallization rate, the activation energy and the fold surface-free energy of the HDPE polymer were apparently changed due to the addition of graphene filler in different diameter size. Particularly, the incorporation of all graphene nanoparticles resulted in a greater crystallization temperature of HDPE, but graphene nanoparticles M5 with the smaller diameter increase more the number of heterogeneous crystallization nuclei and this resulted in a higher crystallization rate. (C) 2016 Elsevier B.V. All rights reserved.