Natural fibres from agricultural residues and forest products mainly consist of lingocellusics. As a result, they are subjected to thermal degradation during composite processing and application. Earlier works on the thermal decomposition of polymer reinforced with natural fiber composites show that they have low thermal properties which are not satisfactory. Hence there is need to improve the thermal properties of polymer composites reinforced with natural fibers. It is in the light of the foregoing that the authors of this book have investigated the effects of orange peel ash on the thermal properties of high density polyethylene. The authors hope that the book will stimulate interest in the study of thermal properties of natural fiber reinforced polymer composites development.