Due to high thermal conductivity and light quality, carbon materials are widely used as additives to enhance thermal conductivity of phase change materials (PCMs). However, existing enhancement effect is very limited because enhancement mechanism and selection criteria of additive is absence. To solve the above problems, a novel model is established from the viewpoint of constituents' micromixing process. The proposed model is validated by the experimental data with calculation deviations lower than 12.8% under 10 wt% (weight fraction) additive amount, while the min deviation is up to 75.9% for existing models. Model analysis shows that the microstructure parameters of the additive including aspect ratio, specific surface area and equivalent thickness, especially the latter two play vital role on the conductivity of composite PCMs. Based on the above criteria, EG-50 (expanded graphite with the size of 282 mu m, expansion 550 time) is selected to enhance thermal conductivity of paraffin based composite PCMs. Experiment results indicate that when adding 3wt%, the optimal thermal conductivity is up to 2.10 W/m K which is 202% as high as the existing max value (1.04 W/m K). (C) 2017 Elsevier Ltd. All rights reserved.