Abstract: On the basis of analysing far infra-red spectra of 22 major tree species of Fujian, for most tree species, the far infra-red radiation which caused the maximum depth of penetration in wood was around 1840 cm~(-1) wave number (5.43μm wave lenth) . Having measured the depth of far infra-red penetration of 22 tree species, we learent that the maximum one was in the tangetial section of "Albizzia kalkora" 0.71 mm; and the minmum one was in the radial section of "Photinia davidsonia" 0.135 mm. We also learnt that depth of the penetration was deeper in tangetial section than in radial section and it varied with the density and moisture content of the wood. The law of wood absorbing the far infra-red radiation corresponded with the Lambert's law of absorption, but the coefficient of absorption was larger in radial section than in tangetial section. According to analysing far infra-red spectra, we suggested that the absorption of wood cellulose and lignine unit is mainly caused by the vibration of the equivlent chains of molecular oxygenic group. These results offered a scientific basis for the study of the fundamentals of wood far infra-red drying.

Key words: Far infra-red, Wood, Depth of penetration