The subgrade is the in situ material upon which the pavement structure is placed. Although there is a tendency to look at pavement performance in terms of pavement structures and mix design alone, the subgrade soils can often be the overriding factor in pavement performance. The subgrade must be able to support loads transmitted from the pavement structure, or in other word, the subgrade must be have high bearing capacity. Bearing capacity is often affected by degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good. Semarang-Purwodadi road is located over poor subgrade of expansive clay soil which have low bearing capacity. As a result, the pavements structures tend to damage in early stage.Some efforts to improve and to strengthen the subgrade have been conducted,however,no one of those efforts can stop the subgrade deformation. Realizing the results of those efforts,Buton Natural Rock Asphalt (Buton-NRA), the natural asphalt found in Buton Island, South-East of Sulawesi,Indonesia,are tried to be used as new material to stabilized expansive clay soils.