Seismic responses of free-field sand ground under horizontal and vertical earthquakes are investigated using a hydraulic earthquake simulator R500B installed on a 450 g-ton geotechnical centrifuge. The test is conducted under a centrifugal gravity field of 30 g with a minimum horizontal seismic acceleration of 20 g and vertical seismic acceleration of 10 g. Several accelerometers are used to measure the seismic responses of the free-field sand ground. The test result shows the seismic difference between the combined responses and the independent responses. The coupled seismic effects on the sand ground should be considered in design