Jute fibres, an environmentally and ecologically friendly product, were chemically modified by treatment with 1.26 M (5 wt%) solution of NaOH solution at room temperature for 2, 4 and 8 h. The above samples were characterized by small angle X-ray scattering (SAXS) technique and XRD for macromolecular and microstructural parameters of fibres before and after alkali treatment where as FT-IR and SEM was used for fine structural details and morphological studies of the fibres. Differential scanning calorimetry (DSC) and instron 1185 analyzed thermal and mechanical behaviour of the fibres. Comparison and analysis of results confirmed some changes in the macromolecular structure and microstructure of the fibres after chemical treatment due to swelling of macromolecules and removal of some non-crystalline constituents of the fibres. The findings conclude that change in crystallinity developed after alkali treatment resulting improvement in mechanical strength of the fibres. However, the removal of structural constituents after alkali treatment leads the thermal decomposition temperature of the cellulose went down to 360.62 degrees C after 8 h alkali treatment from 365.26 degrees C for raw jute fibre.