Abstract Biodegradable composites reinforced with bagasse fibre before and after alkali treatments were prepared, and mechanical properties were investigated. Mechanical properties of the composites made from alkali treated fibres were superior to the untreated fibres. Composites of 1% NaOH solution treated fibres showed maximum improvement. Approximately 13% improvement in tensile strength, 14% in flexural strength and 30% in impact strength had been found, respectively. After alkali treatment, increase in strength and aspect ratio of the fibre contributed to the enhancement in the mechanical properties of the composites. SEM observations on the fracture surface of composites showed that the surface modification of the fibre occurred and improved fibre–matrix adhesion.