In order to study the shear strength characteristics of an assembly of many Expanded Polystyrene beads,direct shear tests were carried out to investigate the relationship between shear stress and shear displacement under different normal stress conditions.The effects of particle size and compressibility on shear strength were analyzed for the beads with particle sizes of 1mm,3 mm and 5 mm.The results show that the shear strength of an assembly of many EPS beads decreases with the increase of particle sizes.The cohesion of the assembly decreases and the internal friction angle decreases from 8.1 kPa to 2.75 kPa when the particle sizes of EPS increases from 1 mm to 5 mm.The reason is that the compression of an assembly of EPS beads increases with the increase of particle sizes.