This paper presents the laboratories results for California Bearing Ratio (CBR) value of road sub-base aggregate mixed with coal bottom ash. The Class-F coal bottom ash used in this study are from Jimah Power Plant, Port Dickson, Negeri Sembilan, Malaysia, meanwhile the road sub-base aggregate used in this study from Hanson Malaysia based on gradation limits from Public Works Department Malaysia (JKR) Standard Specification for Road Works Section 4: Flexible Pavement. The first objective of this study is to determine the CBR value for road sub-base mixed with 3%, 5%, 10%, 15% and 20% of coal bottom ash. The second objective is to determine the optimum concentration percentage of coal bottom ash based on the CBR value for soaked and unsoaked condition. This study involved the California Bearing Ratio (CBR) test for soaked condition and unsoaked condition test to determine the strength of CBR value for road sub-base aggregate. The stabilized of road sub-base aggregate mixed with coal bottom ash results were compared to the control sample of road sub-base aggregate accordingly to Malaysia Public Work Department (JKR) from Standard Specification for Road Works Section 4: Flexible Pavement for road work specification. The laboratory result shows, the optimum concentration of coal bottom ash as additives in road sub-base aggregate is about 9%. The addition of 9% bottom ash to the road sub-base aggregate were improve the CBR value for unsoaked condition and slightly decrease of CBR value for soaked condition. However the result of CBR value for both condition were fulfil the requirement for road sub-base aggregate from Public Works Department Malaysia (JKR) for Standard Specification for Road Works whereas the CBR value for road sub-base more than 30 percent. These study shows, bottom ash more effective used as additives to the road sub-base aggregate to enhance the CBR value.