Abstract: To analyze the interfacial compatibility mechanism in wood flour/polymer composites, the temperature spectra and frequency spectra of polar wood flour/polypropylene composites with different loading levels of silane coupling agent were researched through dielectric relaxation process and then the apparent activation energy and thermodynamic quantity in dielectric relaxation process were calculated. The results show that there is relaxation process in poplar wood flour/polypropylene composites with silane coupling agent which is based on reorientation of the methynol groups in amorphous region of wood cell wall. The dielectric relaxation strength decreases first then slowly increases with silane coupling agent loading. The distribution peaks of relaxation time are high and narrow at the beginning then are broader and lower, finally narrower and higher again with increasing silane coupling agent. The values of apparent activation energy, activation enthalpy, activation free energy and activation entropy tend to increase first and then decrease with increasing silane coupling agent. Apparent activation energy and activation enthalpy of 2.0% silane coupling agent modified composites show the maximal value among all the tested conditions, which is 28.12 kJ/mol and 26.35 kJ/mol,respectively, more than twice and 117.9% as likely to samples without coupling agent. It suggestes that the reorientation of the methylol groups becomes very difficult under the strong hindrance. The compatibility between poplar wood flour and polypropylene plastic is better, and the internal bonding and interface strength is stronger, property is more stable.