Recently, interest in environmentally friendly materials is increasing with the issue of environmental pollution due to plastic waste. Accordingly, studies of natural fiber composite materials have been continuously done, but studies on strength and interfacial adhesion of environmentally friendly composite materials are insufficient. There are various ways to improve strength or interfacial adhesion, one of which is to add nanoparticles to improve mechanical properties. In this study, additives are manufactured using eco-friendly and inexpensive natural materials, replacing the existing nanomaterials (CNF, CNT, Graphene). Natural fiber composite materials were prepared using the prepared additives and cellulose nanofibers, and mechanical properties were evaluated and analyzed. First, seaweed was selected as a natural material to be prepared as an additive. Cellulose was extracted by removing impurities, oxygen-based bleaching, cellulose extraction, neutralization and filtering, and pH measurement process. Cellulose characteristics appeared at extracted cellulose through FT-IR and XRD tests, and SEM image analysis confirmed the fiber shape having a diameter of 2~3㎛. Mechanical properties were tested for tensile, shear, three-point bending, and impact tests. As a result, the mechanical properties improved due to the addition of seaweed cellulose and cellulose nanofibers. At 0.3%, it had the highest mechanical properties and showed an increased tendency than the JFRP modulus. As a result of confirming the fiber surface through SEM image, the additive adhered to the fiber surface to improve the roughness. so that, This confirmed that the interfacial adhesion was improved. In the case of impact behavior, the impact strength was improved due to the additive, and the impact energy was dispersed in the additive, thereby improving the absorption energy. The seaweed cellulose developed through this study is effective in improving the mechanical properties of natural fiber composites, and it is expected to be applicable to various fields through subsequent studies