The improvements achieved in the engineering properties of weak clayey soil are a function of various influencing agents, among which the gradation of stabilizers is one. In this study, the impact of variation in scoria gradation on the compaction characteristics, strength properties, and hydraulic conductivity of clayey subgrade was investigated. Four different gradations of volcanic scoria (well-graded, gap-graded, uniformly graded gravel-sized, and uniformly graded sand-sized scoria) were considered for the experimental investigation. Samples of clay soil were tested for compaction characteristics, California bearing ratio, and hydraulic conductivity before and after treatment for each gradation. Finding of the study revealed that the gradation of scoria as a stabilizing material has a considerable influence on the strength, compaction characteristics and permeability of the clay. The uniformly graded sand-sized scoria was found to be the gradation type with the highest performance in modifying the California bearing ratio and maximum dry density of the clayey subgrade. In contrast, the least performing gradation of scoria was the gap-graded one for all considered mix ratios. With regard to permeability, the highest degree of improvement in hydraulic conductivity of the clay was obtained when uniformly graded gravel-sized samples are blended with the weak clay.