Fiber-matrix interaction at the interphase is one very important property that is of great concern to all polymer scientists involved in polymer composites. Many of the failures can be traced to the type of interfacial interaction existing in the composites. That is why highlighting the factors that dictate the type of and the extent of interactions at the interphases become very necessary. Natural fiber polymer composites have found application in many fields of human endeavors. To continue this growth being experienced, the factors that determine the formation of good interaction at the interphase most be understood, so that they can be manipulated for a better result.

Natural fiber composites are designed for different purposes including structural and non-structural ones. These natural fiber composites vary greatly in their properties including mechanical properties. Mechanical properties which include the tensile and flexural properties are highly dependent on factors such as matrix type, filler type, processing, post processing treatment and many more, factors which are quite application specific. However, many research works develop their natural fiber composite before considering the possible applications. This chapter intends to X-ray the factors that affect the mechanical properties as it relates to structural and biomedical applications and suggest ways of improving the mechanical properties.