This study focuses on the change in the fine structure and tensile properties of ramie fibres by the treatment for crystalline conversion. Through the conversion from cellulose I into II, IIII and IVI, the integral crystallinity index and the crystallite size decreased and the internal surface area increased while Young’s modulus of the fibre decreased and the ultimate strain increased. Two phase parallel, series and parallel-series models which comprise two kinds of springs with different constants representing the crystalline and amorphous components, respectively, were applied to the tensile force-strain relationship of the fibres. The parallel model could not be applied to the samples because the crystal lattice strains were smaller than the fibre strains; the former strains were less than one fourth of the latter ones, at any given tensile load. On the basis of the series model, Young’s modulus of crystalline component of cellulose IIII was the highest and the next was that of cellulose I, followed by II and IVI. Young’s modulus of amorphous component of cellulose I was the highest and the next was that of II, followed by IIII and IVI. Applying a parallel-series model, the values for Young’s moduli of crystalline and amorphous components were found to depend both on the feature of crystalline component and the density of amorphous region.