Sedimentation in river triggered by soil erosion has an impact on the ecosystem services. Application of natural biomass on residual as biodegradable material has been of great attention due to its environmental friendliness. In this study, oil palm empty fruit bunch (OPEFB) and seaweed are applied as a composite to evaluate the water holding capacity and thickness swelling as soil stabiliser. The alkaline treatment is introduced to improve the surface bonding of fibres with binder. The dimensional stability test was conducted according to standard ASTM D 5229 to obtain average water absorption and thickness swelling. The average water absorption and thickness swelling for the nontreated composite are 297.07% (± 6.47) and 7.78% (± 3.34) respectively, while for the alkaline treated composite, it demonstrates lower water absorption (222.60% (± 4.38)) but higher thickness swelling 18.23% (± 4.23). This showed that both of the composites have high water content mainly due to dimensional stability. Scanning electron microscopy (SEM) micrographs display better interface adhesion between OPEFB fibre and seaweed after alkaline treatment compare to non-treated composite. Fibre breakage was observed for the non-treated composite, suggesting that it is incapable to withstand the tension created by water during the immersion.