India as a tropical agricultural country has great potential to develop and use fiber derived from agricultural waste. Natural fibers are an important by-product of extraction process and they can be used as reinforcement in composite products. Composites are developed with unsaturated polyester resin as the matrix with natural fiber as the reinforcement. The results show decreased strength and modulus with increasing the fiber volume fraction. This indicates ineffective stress transfer between the fiber and matrix due to lower adhesion. It is necessary to bring a hydrophobic nature to the fibers by suitable chemical treatments in order to develop composites with improved mechanical properties. In these review papers, different types of natural fibers are subjected to a variety of physical and chemical treatments. The types of treatments studied in these papers include Physical treatments such as beating and heating, and chemical treatments like alkalization, silane, acetylation and benzoylation. The effects of these treatments on mechanical properties of the composites are analyzed. Fractures are analyzed by using the scanning electron microscopy (SEM). Analysis by FTIR and DMA showed that physico-chemical changes of surfaces of treated natural fibers. In general, treatments to the fibers can significantly improve adhesion and reduce water absorption, thereby improving mechanical properties of the composites. The purpose of this review paper is to summarize the research work done on various pretreatments in the preparation of natural fiber reinforced composites and to highlight the potential use of natural fiber reinforced polymer composites in industry and its potential to replace the synthetic fiber composite and conventional materials in the future.