Plastic mulch is a layer of material applied to the surface of the soil to conserve soil moisture, optimize fertilizer usage, and reduce weed growth. However, mulch made from low-density polyethylene (LDPE) causes problems, after its use, because it cannot decompose naturally. Moreover, photodegradation by ultraviolet (UV) radiation causes LDPE plastic to break down into microplastic. On the other hand, lignin has been studied to have good potential as a natural UV protection material. Coconut coir is a natural fiber that contains high lignin of about 32–45%. Compounding LDPE with coconut coir powder is expected to produce a biodegradable plastic composite, and more resistant to UV light exposure. In this study, coconut coir powder was utilized as filler in LDPE-based composite by 1, 3, 5, 10, 15, and 20% of total weight, and poly (ethylene-alt-maleic anhydride) at 2% of total weight (at most) was used as compatibilizer. The materials were mixed in the rheomixer (rotor rotating speed of 80 rpm, at 120 °C for 10 min). The plastic bio-composites were characterized by mechanical, UV light resistance, and biodegradability properties.