In the paper, the cellulose as a fiber-forming component of wood (natural composite) has been studied. The authors put forward a technique for estimating fluctuations of cellulose microstructure in the wood through monitoring the potential difference of the thermal polarization that arises in the samples placed into an inhomogeneous temperature field with a constant temperature gradient. Formalized simulation was used for an analysis of experimental results. The proposed technique made it possible to establish that the percent of the large-sized cellulose crystallites in the wood grew with increasing smoothly temperature gradient. Similar dynamics is not typical of linear crystalline polymers whose polarization decreases with growing temperature. The obtained effect can be assigned to the fact that natural wood exhibits heterogeneous structure.