Recently, many agricultural waste were left for open burning or as landfill in many agricultural land, for example, coconut shells. Besides, expanded polystyrene (EPS) foam is one of the most used plastic products which mostly ends up in landfilling as many of its products were made for one-time use only. In order to reduce these wastes, this project focuses on the preparation of a new material that is coconut shell powder (CSP)-filled EPS wood plastic composite (WPC). This research investigates the effects of CSP content and alkaline treatment with sodium hydroxide (NaOH) on mechanical and water absorption properties of EPS/CSP WPCs. The mechanical properties, such as tensile strength, Young’s modulus and flexural modulus, increased as the CSP content increased. However, elongation at break and flexural strength decreased with CSP content. Moreover, water absorption of EPS/CSP WPCs showed an increasing trend with CSP content. After alkali treatment, the mechanical properties of EPS/CSP WPCs were enhanced, but the water absorption properties of treated EPS/CSP WPCs were reduced due to the reduction of the –OH groups of CSP. Fourier transform infrared analysis was carried out to measure the functional groups of untreated and treated CSP. The fracture surface of CSP, untreated and treated EPS/CSP WPCs were examined by scanning electron microscopy.