The thickness of road depends on geotechnical properties of subgrade soil and traffic intensity. The clayey soil is not suitable for subgrade due to its swelling characteristics. The soil having more liquid limit (LL), lower Maximum dry density (MDD) and higher Optimum moisture content (OMC) is suitable for subgrade of road. The clayey soil having more LL, lower MDD and higher OMC. Therefore clayey soil is not suitable as subgrade soil of roads. This paper deals with stabilization of clayey soil by using fly ash and RBI Grade 81 to improve the geotechnical properties of soil. The fly ash is a thermal waste and RBI Grade 81 is a chemical soil stabilizer. In the laboratory standard Proctor test (SPT), Atterbergs limits and differential free swelling index (DFS) test for different proportions of soil, fly ash and RBI Grade 81conducted. The results show that, the LL, MDD, OMC and DFS index of clayey soil improved considerably. The LL of untreated soil is 67% and it reduces to 46% for mix of soil: fly ash: RBI Grade 81 for 76:20:04 proportion. The DFS of untreated soil is 65% and it reduces to 40% for addition of fly ash and RBI Grade 81
