In this paper fundamental sound absorption characteristics of expanded polystyrene beads with several geometries are reported when they are packed in some thick layers with and without back air space. The normal incidence sound absorption coefficients, which were obtained by the experiments using two-microphone impedance tube, were evaluated comparing to theoretical estimations. Experimental data were also compared to the similar conditions but hard material particles like glass beads whose acoustic property was investigated by Sakamoto et. al. in a preceding study. Then following results were obtained; packed layer with expanded polystyrene beads has almost equal sound absorption coefficient to the similar layer with hard and solid material: numerical estimation of sound absorption coefficient is valid enough except for the case having large back air space: lowering peak frequency of sound absorption coefficient is available by overfilling and compressing particles. The authors also consider that applying soft, light, and low cost material to the sound absorbing structures is very meaningful. Because human damage caused by falling of wall and ceiling panel of buildings is avoidable particularly under an earthquake attack.