Abstract: 【Objective】 The purpose of this study was to determine the bound water content and pore size diameter distribution in the swollen cell walls of two fast-growing trees(Cunninghamia lanceolata(Chinese fir)and poplar)using nuclear magnetic resonance(NMR)cryoporometry. The results should be valuable in the selection of particle size for wood modification. 【Method】The T₂ signal amplification of specimens at normal temperature and different frozen temperatures was compared to avoid the phenomenon that bound water content is lower than the theoretical value determined by T₂ relaxation distributions at normal temperature. The maximum moisture content and pore size diameter distributions of the swollen cell wall were determined from the total signals of samples according to the Gibbs-Thomson equation, and inversion calculations were omitted. 【Results】 The saturated bound water content of Chinese fir and poplar was approximately 38%, which was clearly higher than the fiber saturation point(approximately 30%)measured by the extrapolation method, but was in accordance with solute exclusion, porous plate and centrifugal methods. The proportion of pore diameters smaller than 1.59 nm was approximately 75%, and the proportion of pore diameters larger than 4.56 nm was below 6%; this result was also in accordance with solute exclusion and spectral methods. 【Conclusion】 The bound water content and pore size diameter distributions of the cell wall could be conveniently and accurately determined by NMR cryoporometry, which could be used in the selection of modification agents and evaluation of modification effects.