Coal fly ash (FA) and basic oxygen furnace (BOF) slag are the by-products of coal combustion in power stations and steel plants, respectively. The disposal of these wastes would require landfill space and can cause environmental pollution. In order to avoid disposal, the use of FA and BOF slag as sub-base material in highway pavement was investigated. The first part of this research was the characterization of FA and BOF in terms of their chemical and mineralogical compositions as well as the microstructural analysis. This is followed by the preparation of soil mixtures containing different proportions of kaolin, FA, BOF slag (0%, 15%, 20% and 25% by weight) and lime at a constant ratio (5%). Kaolin was used to represent the natural soil. The soil mixtures were cured for 1, 7 and 28 days and some were cured 56 days. At each curing age, the dry unit weight (gamma(dmax)), the unconfined compressive strength (UCS) and California bearing ratio (CBR) were determined. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were also used to examine the changes in the phase-mineral composition and microstructure properties, respectively. The results show that the maximum dry unit weight values were observed in the mixture consisting of 25% FA, 15% BOF slag and 5% lime. The XRD analyses of kaolin soils containing, FA, BOF slag and lime exhibited the formation of hydrated products such as portlandite (C-H), calcium silicate hydrates (C-S-H) and calcium aluminium silicate hydrate (C-A-S-H), which increase with the age curing. According to results obtained, FA and BOF slag can be used in road base and sub-base materials.