Seismic safety of buried structures has become increasingly important over the past two decades, especially after the destructive earthquakes such as in Kobe, Japan (1995), Kocaeli, Turkey (1999) and Chi-Chi, Taiwan (1999). Some of the embedded structures including pipelines, subways and tunnels collapsed or suffered severe damage in those earthquakes due to inappropriate design. The main difficulty in seismic design is the incorporation of soil-structure interaction effect governed by the relative stiffness (flexibility ratio) between the soil and the embedded structure. This study aims to clarify the effect of flexibility ratio on the dynamic response of rectangular structures buried in dry sand. For that purpose, a series of dynamic centrifuge tests were conducted on two types of box-shaped models with different rigidities under various harmonic motions. The results reveal that the magnitude of dynamic lateral earth pressure and sidewall deformation is highly dependent on the flexibility ratio of the embedded structure. Based on the flexibility ratios, racking deformations observed in centrifuge tests and racking deformations estimated through analytical approaches were evaluated in a comparative manner.