To clarify the interaction between the wood and polymer in wood flour/polymer composites, the dielectric constant and dielectric loss factor values were measured for poplar (Populus tomentasa Carr.) wood flour/polypropylene (PP) composites with different wood contents. The dielectric relaxation strength, distribution of relaxation time, and activation thermodynamic quantities of the dielectric relaxation caused by the reorientation of the methylol groups (CH2OH) in the amorphous region of the wood cell wall were then calculated. The results show that the dielectric relaxation strength changed very little below a wood content of 40% and began to increase above that value; this was due to the strong hindrance of PP to the reorientation of methylol groups and, therefore, suggests a close interaction between the wood and PP below a wood content of 40%. The low distribution coefficient of the relaxation time at extremely low temperatures below a 40% wood content was also found; this indicated that some groups in the wood could not move because of the influence of PP. The apparent activation energy increased with wood contents below 40% and then decreased; this further confirmed the optimal interfacial compatibility at a 40% wood content in the wood flour/PP composites in the absence of additives. This result was consistent with the results we obtained by a stress-relaxation approach. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013