Many pile foundations were severely damaged in the reclaimed land during the Hyogo-ken Nanbu Earthquake, January 17, 1995. The objective of this study is to investigate the contribution of axial-force fluctuation in piles to the responses of a pile foundation in the liquefied soil during earthquake. Earthquake response analyses of a structure on a pile foundation are conducted using the numerical model that takes into account the relationship between bending moment and curvature dependent on axial force. These analytical results indicate that axial-force fluctuation in piles affects the elastic-plastic behavior of a pile foundation.