Abstract: In ocean engineering hydrodynamics, the fluid is mostlyassumed with constant density. However, the density does change in oceansowing to variations in salinity and temperature in the vertical directionthrough the water. Internal waves are abundant in stratified oceans. Suchwaves are known to produce significant effects on hydrodynamiccharacteristics of offshore structures. The aim of this paper is to presentthe experimental results on the interaction of internal waves in astratified fluid with a semi-submersible platform. The wavelength, thewaveheight as well as the wave period of the internal wave generated by therocker-flap wave maker are measured; the relations on both the wavelengthand the wave height with the wave period are obtained. Both the laser andthe clinometer are applied to measure the surge and pitch responses of thesemi-submersible platform due to the internal waves, and the relations onthe amplitudes of the surge and pitch motions for the platform with the waveperiods of internal waves are obtained. Results show that the influence ofinternal waves on the motion responses of the semi-submersible platforms cannot be neglected in their designs and applications. In particular, thesum-frequency effect of the motion response of the semi-submersible platformdue to internal waves is found over a wide range of wave periods.