This study used the Taguchi method to determine optimum conditions for Gilsonite and glass fibers as modifier materials in hot mix asphalt (HMA) by conducting the mixed Mode I/III fracture test. These modifier materials were used under different experimental parameters. The parameters and their levels were angle of fracture ( and 45°), nominal maximum aggregate size (9.5, 12.5, and 19 mm), air void content (4, 7, and 10%), Gilsonite content (6, 12, and 18%, as percentage of binder weight), glass fiber content (0.1, 0.2, and 0.3%, as percentage of aggregate weight), glass fiber length (3, 8, and 12 mm), and experimental temperature ( , , and ). The optimum conditions for experimental parameters were fracture angle ( ), air void content (4%), nominal maximum aggregate size (9.5 mm), Gilsonite content (6%), glass fiber weight (0.3%), glass fiber length (12 mm), and experimental temperature ( ).