A procedure to estimate the seismic sliding displacement of the gravity type quaywall was studied by the shaking table test of the underwater model which consists of the mortar block and backfilling sand. According to the tests, the response acceleations of the model block and the backfilling sand wedge of the active failure decreased instantaneously on set of sliding and remained the same levels during sliding. The measured resultant earth pressure to act on the model block decreased after the beginning of sliding, and remained the same level during sliding. The measured coefficients of earth pressure at the start of sliding were in agreement with the sum of the coefficient of earth pressure at rest and the remainder of the coefficient between Mononobe-Okabe formula and Coulomb formula. The measured coefficients of earth pressure during sliding were rather smaller than the coefficient of earth pressure during earthquakes of Mononobe-Okabe formula. Based on test results two kinds of procedures to estimate the seismic sliding displacement which are the rigid body sliding model and the FEM model were presented. The calculated displacements were in good agreement with experimental ones of the first sliding in the cases of no water, and was rather larger than the experimental ones in the underwater cases.