In order to investigate the cyclic stress-strain and strength characteristics of clays at the site where KANSAI International Airport is planned, both resonant cyclic triaxial tests and cyclic simple shear tests are carried out using undisturbed samples taken from the site. Main results are summarized as follows:
　1) The vibratory shear moduli at small amplitude of shear strain, Go measured by resonant column tests on the specimen subjected to 24 hour consolidation under
the in-situ effective overburden stress, coincide with those determined by in-situ seismic surveys at far as the samples in this site are concerned.
　2) Go can be estimated by the proposed equation considering the consolidation yield stress.
　3) Go of normally consolidated clays increases proportionally to the consolidation pressure. This increment is more pronounced in the clay with low plasticity than the clay having a high plasticity index.
　4) The strain-dependence of vibratory shear moduli is significant for the clay with low plasticity.
　5) The damping ratio has a correlation with the plasticity index. Based on the present test results, the clay with low plasticity has a higher damping ratio than the clay with high plasticity.
　6) Both the cyclic strength and the cyclic strength ratio of clays at the site are large than those of other normally consolidated clays.