In this study a thorough analysis of physical and fine structure of hemp fibre bundles, namely surface topography, diameter, cellulose content and crystallinity index, have been presented. The fibre bundles have been alkalised and physical and mechanical properties analysed. Alkalisation was found to change the surface topography of fibre bundles and the diameter decreased with increased concentration of caustic soda. Cellulose content increase slightly at lower NaCH concentrations and decrease at higher NaOH concentrations. The crystallinity index decrease with increase in caustic soda concentration up to 0.24% NaOH beyond which, it decreases with increase in NaOH concentration. It was also found that the tensile strength and stiffness increases with increase in the concentration of NaOH up to a limit. Tensile strength and Young's modulus increase with decrease in cellulose content, while crystalline cellulose decreases slightly but with improved crystalline packing order resulting in increased mechanical properties. Similar observations are elucidated by the crystallinity index. Alkalised hemp fibre bundles were found to exhibit a similar specific stiffness to steel, E-glass and Kevlar 29 fibres. The results also show that crystallinity index obtained following alkalisation has a reverse correlation to the mechanical properties. Stiffer alkalised hemp fibre bundles are suitable candidates as reinforcements to replace synthetic fibres. The improvement in mechanical properties of alkali treated hemp fibre bundles confirms their use as reinforcement materials. (c) 2006 Springer Science + Business Media, Inc.