The use of natural fibres at high percentages of loading in thermoplastic composites for the production of sustainable and green materials in consumer goods, furniture, automotive industry and construction industry is encourage. Several studies have been conducted by many researchers to improve the mechanical properties of the fibres and the fibre-matrix interface for better bonding and load transfer especially when high fibre loading is used. The natural fibre hydrophilic properties make the poor interface and poor resistance to moisture absorption when used to reinforce hydrophobic matrices. Therefore, this study investigates the effects of different surface treatment of kenaf bast fibre on the flexural strength of kenaf polyethylene composite (KPC). These composites, made using high-density polyethylene (HDPE) as the matrix polymer, kenaf core and kenaf bast fibre as the reinforcing filler at different percentages of filler and maleic anhydride grafted polyethylene (MAPE) as compatibilizing agents. Overall, KPCs with bast fibre treated with 0.06M MgCl2 and 0.06M NaOH enhanced the flexural strength of the composites as compared to untreated bast fibre in the composite. Besides, the flexural properties of the KPCs significantly decreased with increasing kenaf bast fibre content, due to the reduction of interface bond between the fibre and matrix.