This paper introduces the waste denim fiber as a modifier for bitumen used in the flexible road construction. The sources, chemical components, and properties of denim fiber are discussed. Methods of adding different types of fibers into bitumen and asphalt mixtures are also presented. In addition to that, a review of the effects of cellulose fiber as the main component part of denim fiber in bitumen and asphalt mixture is summarized. The motivations of using denim fiber as a modifier for base bitumen are also highlighted, such as the mechanical properties, lower water content compared to other natural fibers and the sustainability in terms of low cost and mitigate the environmental problem in the same time. Furthermore, Challenges of using denim fiber for bitumen modification are reported. It is expected that a long lasting, cost effective and environmentally friendly denim fiber modified asphalt and asphalt mixtures, which are rut and fatigue resistant will be developed in near future. This will decrease the annual expenses of flexible pavements maintenance required around the world. © IAEME Publication.