The national demand for asphalt is around 1.2 million tons per year. However, PT Pertamina can only supply around 0.6 million tons, so the rest must be imported. To minimize imports, the Indonesian government has instructed the stakeholders to maximize the use of natural asphalt in Indonesia, specifically Asbuton from Buton Island. One study that was conducted compared the optimum asphalt content and stiffness modulus characteristics of modified Asbuton mixtures and unmodified asphalt mixtures. This study employed an experimental method. Marshall testing was carried out to determine the optimum asphalt content of two types of asphalt mixtures: one without Asbuton and one with Asbuton B 50/30. The results were then compared. For the mixture with Asbuton B 50/30 added, the levels added were 10 and 15%. The stiffness modulus test was conducted using the ITSM method. The required input data for this study are the aggregate test, the Asbuton test, the Marshall properties test, and the stiffness modulus test. The Marshall test revealed an optimum asphalt content of 5.5% for the asphalt mixture with 60/70 penetration, as well as 6% and 7% for the modified asphalt mixture with Asbuton B 50/30 added at 10% and 15%, respectively. The stiffness modulus values obtained from the ITSM test were 6.277 MPa for the asphalt mixture with 60/70 penetration and 8.098 and 8.539 MPa, respectively, for the modified asphalt mixture with 10 and 15% Asbuton B 50/30.