The present study focused on laboratory investigation of fly ash mixed with expanded polystyrene (EPS) beads. Different sizes of EPS beads were mixed with fly ash to study the effect on maximum dry unit weight, cohesion, angle of internal friction, California bearing ratio (CBR) value, coefficient of consolidation, and permeability. The different EPS beads sizes used in the present study are 1 mm, 2 mm, 3 mm, 4 mm, and 6 mm with 28 kg/m3, 19 kg/m3, 16 kg/m3, 11 kg/m3 and 6 kg/m3 densities, respectively. The effects of EPS beads sizes on geotechnical properties of fly ash mixture were investigated. The constant EPS beads content of 0.6% was considered for comparing the effect of different sizes of EPS beads in the fly ash. The smaller EPS beads–fly ash mix was observed to be suitable for improving the shear strength and bigger EPS beads were good enough to reduce the unit weight of fly ash–beads mix. The CBR value and permeability of fly ash–EPS beads mixtures were increased with a decrease in the size of EPS beads. The coefficient of consolidation increased up to 4-mm size of beads and then decreased for 6-mm size of beads. The fly ash reinforced with 0.6% EPS beads of different sizes, specifically 1-mm and 2-mm size, can be useful for constructions on weak soils. Each size of EPS beads has benefits as per the requirements in engineering applications and field conditions.