Where the self weight of the overall RC(Reinforced Concrete) or PC(Precast Concrete) floor system needs to be considered, the quantity of in-situ concrete required can be reduced by the application of polystyrene void forms to slab. In this study, flexural behaviors of void RC and PC slab with polystyrene form were evaluated to understand the effect on the self weight reduction of the slab. This paper presents experimental works and finite element analysis on the bending of the void slabs. Six specimens were tested. The main parameters of experiments were the slab types (RC and PC), the types and arrangement details of polystyrene form, and the thickness of slab. Structural behaviors of void slabs were evaluated on the basis of failure mode, load-displacement curve, and ultimate strengths. Test results indicated that nominal strengths by ACI code agreed well with experimental results. The results of analysis were compared with the results of test for estimating the validity of analysis model.