In this study hemp-thermoset sandwich composites were prepared by using unsaturated polyester resin. To improve fibre matrix adhesion alkalization and acetylation treatment on the fibres were carried out. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to investigate the changes of fibres chemical constituents after treatment. Mechanical properties such as flexural strength, flexural modulus and strain at break of untreated and treated fibre composites were studied and compared. The incorporation of the alkalized fibres resulted in composites better flexural properties and 8% treatment showed best results. Thermal stability of the fibre was also enhanced for the treated fibre. In general, thermal and mechanical properties of the treated fibre and its composites were superior to the untreated fibres.