Numerical simulations of unsaturated soil triaxial tests were performed using the porous media theory and a simplified elasto-plastic constitutive model for sand. In the tests, cyclic shear strain was applied to fine clean sand with the same dry density but with different initial suction under undrained conditions. The zero skeleton stress state (i.e. liquefaction) for unsaturated sand was achieved when both the pore air pressure and the water pressure built up to the initial total pressure. Therefore, it is necessary to consider pore air pressure for the proper simulation of liquefaction of unsaturated sand. In the simulation, three phases with a soil skeleton, pore water, and pore air were considered on the basis of porous media theory. The simplified elasto-plastic constitutive model for the soil skeleton was based on the non-associated flow rule and the nonlinear kinematic hardening rule. The simulations provided a satisfactory reproduction of the triaxial test results in the cases where the initial degree of saturation was higher than about 70%.