Welding of the Submerged Arch Welding (SAW) type produces a by-product in the form of slag. SAW welding is widely used in the steel fabrication and ship building industries. In order to reduce the accumulation of SAW slag waste that can adversely affect the environment, this waste needs to be utilized sustainably. Currently SAW slag is used in the manufacture of concrete, but research has not been found related to road pavement structures. This paper focuses on analyzing the CBR values of base course substituted by SAW slag. The base course material used refers to the Spesifikasi Umum Bina Marga Revision 2 of 2018. The contents of SAW slag substituted on the basecoarse are 0, 5, 10, 15, and 20%. The tests carried out were Specific Gravity Test, Bulk Density Test, Los Angeles Abrasion Test, Modified Compaction Proctor, and California Bearing Ratio Test (Soaked CBR Test). The CBR test results showed an increase in the CBR value for each blow (10 blows, 30 blows, and 65 blows) up to the 15% and back down at the 20% content of SAW slag. CBR Design obtained from the relationship of CBR graph with compaction shows an increase of 23.5% at 15% SAW slag content. Therefore, the use of SAW slag at 15% by weight is the optimum content of SAW slag which can be recommended as a substitute material for a base course.