Recent laboratory tests suggested that pore air plays an important role during liquefaction of unsaturated soil. In this study effect of pore air pressure on cyclic behavior of unsaturated sandy soil is discussed using three-phase porous media theory. Numerical simulations of the undrained cyclic triaxial tests with unsaturated sandy soils are performed using the porous media theory and simplified elasto-plastic constitutive model. The effect of suction is taken into account by skeleton stress only. The effect of pore air pressure on cyclic behavior of unsaturated sandy soil is discussed through the simulations with three-phase and simplified two-phase analyses. Two-phase analysis assumed that pore air pressure equals to pore water pressure under undrained condition. Three-phase analysis reproduced the cyclic behavior of unsaturated sandy soil with low initial suction. On the contrary two-phase analysis overestimated the volumetric strain due to dilatancy because two-phase analysis assumed equivalent compressibility of pore air and water. Three-phase analysis is applicable for precise prediction of seismic behavior of unsaturated sandy soil.