Abstract: Fractal theory was first introduced into the description for ultrastructure of wood cell wall in this study. The variability of the fractal dimensions（D）values of the nanofibril in wood cell wall in both radial and tangential directions was studied by taking the mean microfibril angles θ in S2 layer of the wood cell wall determined by polarized light microscopy as the principle parameter. The results showed that the D values in radial direction of the samples close to the pith were the maximum, which declined in a wavy pattern toward the bark direction from the quick descent to slow descent stages. The variation of D values in tangential direction was not obvious, there were no distinct stage characteristics in the wavy descending course.