This study assessed the morphology and chemical composition of coir coconut husk carbon fiber, as well as the impact of fiber diameters on the physical and mechanical properties of polylactic acid composites. Researchers are studying polylactide acid, a biodegradable material. This eco-friendly material’s excellent features, generated from sustainable and renewable sources, have drawn many people. Malaysia’s high coconut fiber output made coir husk a popular commodity. Coconut fibers are lignin, cellulose, and hemicellulose. Alkaline treatment eliminates hemicellulose, oil, wax, and other contaminants from coir fibers and removes lignin. Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy were used to examine the treated coconut fibers’ chemical modification analysis and morphology. Coconut coir husk was carbonized to produce carbon fiber using a furnace operated at 300°C for 2 hours. Fiber and polylactic acid were mixed in different fiber sizes (0, 53 μm, 75 μm, and 212 μm) via extrusion and injection processing techniques. The results showed that the alkali treatment reduced the hydroxyl (-OH) group and separated the area from the carbonyl (C=O) group of coconut coir husk, which changed the filler’s hydrophilicity. The fiber size of 212 μm was discovered to have the highest tensile and flexural strength values. According to testing, the modified material structure had a better surface fill-matrix bond. Thus, generalized fiber sizing and characterization methods were developed. Regardless of the matrix, this method can characterize natural fiber strength and interfacial shear strength of varied diameters and solid contents.