For a long time, bitumen has been utilized as the essential material for asphalt pavement construction. The factors of increasing axle loads, increasing traffic movement, critical climate conditions and many forms failures in construction have steered many researchers to seek some methods to enhance the asphalt binder properties. Even though various types of modifiers have been utilized in strengthening asphalt concrete, fibers have attracted the most attention due to their high and desirable characteristics. It is realized that the good distribution of the modifier in asphalt binder can generate a strong network in the interior structure of the blend, causing bitumen mastic to be more coherent. In this study, a laboratory investigation of the rheological and physical properties of various grades of bitumen modified by two types of fibers was conducted. Three grades of asphalt were used in this study (60-70 penetration grade, 80-100 penetration grade and PG-76 grade) with two types of fibers with different percentages—cellulose oil palm fiber (COPF) (0.15, 0.3, 0.45, 0.6, and 0.75%) by weight of asphalt and carbon fiber (0.75, 1.25, 1.75, 2.25, and 2.75%) by weight of asphalt. The results showed enhancement in physical performance of the modified bitumen in terms of the decrease in penetration values, as well as a rise in the softening point and viscosity values. The fibers’ modified asphalt binders showed improved rheological properties and can raise the grade of asphalt depending on the base asphalt type.