The improvement of engineering properties of base and sub-base course soil was investigated and studied using steel slag material. For this purpose, an experimental program was developed and performed to investigate the improvement of shear strength and California Bearing Ratio (CBR) of base soil materials by adding variant percentages of steel slag. The testing program were designed depending upon maximum dry density, γdmax., and optimum moisture content, OMC. In addition to that, a field performance was tested and investigated for the treated base soil of a road section of 100 m long and 3.5 m wide. The results of laboratory tests showed that: (a) CBR and shear strength of treated base materials have a considerable increasing when coarse soil-steel slag is used by 20% to 30%; and (b) the values of CBR are greater than that of non-treated base coarse soil when coarse soil-steel slag is used by 20% to 30%. Additionally, the field plate loading test results indicated that the vertical deformation (settlement) and permanent deformation of the treated base course is less than that of non-treated base course.