Using multi-scale instrumental analyses, nanostructural changes due to superheated steam treatments were compared between Japanese cedar (Cryptomeria japonica) and bamboo (Phyllostachys pubescens Mazel). The lower amounts of aromatic carboxylic acids contained in lignin and hemicellulose/lignin in Japanese cedar suppressed not only the formation of pyrolysis products from hemicellulose but also fragmentation of cell walls. Variable temperature solid-state NMR spectra and relaxation time analyses were carried out on the nanostructures of the plant materials in order to examine molecular motions and interaction with bound water for the biomass constituents of the plant materials. The untreated and steam treated bamboos strongly interacted with bound water to change the 1H spin-lattice relaxation time (T1H) values depending on the content of bound water. Because the interaction between the biomass constituents and bound water in Japanese cedar was weaker than that in bamboo, the T1H values of Japanese cedar were reduced less than those of bamboo in the humid conditions. The difference in the interaction with bound water between bamboo and Japanese cedar also affected the amount of the pyrolysis products and the decomposition rate of hemicellulose in the superheated steam treated materials. Furthermore, the pyrolysis products caused little change in the molecular mobility of Japanese cedar although they restrained that of bamboo.