In this book, coir fiber was chemically treated with acrylonitrile to improve the physical and mechanical properties of the coir-PP composites. Physical and mechanical properties of the composites prepared from treated coir were found better than those of untreated ones. The tensile strength, tensile modulus, flexural strength, flexural modulus, and hardness of the chemically treated coir fiber reinforced composites were higher than those of the raw ones. Treated coir-PP composites were found to absorb less amount of water than the untreated ones. To understand why the mechanical properties of composites prepared under different conditions of coir were different, surface morphologies of the tensile fractured surfaces of the specimens were recorded using scanning electron microscopy (SEM). The SEM images clearly revealed that there were fewer fiber agglomerations, micro-voids, and fiber pull out traces for treated coir-PP composite than in the untreated one, indicating that betterdistribution of the fiber into the matrix as well as stronger fiber matrix interfacial adhesion occurred upon treatment of coir.