The concerns of oil depletion and the growing demand for new and eco-friendly alternatives for crude oil asphalt (COA) has prompted utilize of natural bitumen (NB) in pavement field. Iraq has many deposits of NB and the most common types are lake bitumen (LB) and Gilsonite bitumen (GB). This research investigates the performance improvement of LB using GB as a modifier. The modified blends were designed by partially replacing 30%, 35%, and 40% of the weight of LB with GB. Physical and chemical testing were conducted on bitumen specimens, including specific gravity, solubility, softening point, ductility, penetration, viscosity, and X-Ray Diffraction (XRD). In addition, indirect tensile strength test and wheel tracking test were applied on optimal NB-modified mixture and control mixture prepared with COA. Based on the results, modified LB with GB significantly reduced fluidity, increasing rotational viscosity at 135 ºC approximately by 1.7, 2.6, and 4.3 times for the blends with 30%, 35%, and 40% GB, respectively. Also, the stiffness of modified blends was increased, for instance, the penetration value of modified LB with 35% GB reduced by 67.4% compared to base LB (unmodified). Modified LB with 35% GB is the optimal blend as it comply with the specification limits for COA. XRD analysis demonstrated that LB has an amorphous structure similar to COA and unlike GB, which has a crystalline structure. Also, modified LB samples have a crystalline structure, indicating to the dominant influence of GB on the chemical properties of LB. Compared to the control mix, the NB-modified mix showed a higher TSR value and a lower rut depth with improving rate of 13.9 % and 53.9%, respectively. In conclusion, GB modifier with optimal content of 35% makes LB mixture more resistant to moisture damage and less susceptible to rutting failure.