In order to clarify an optimum improvement dimension for the permeable-grouting method as a liquefaction countermeasure, a series of shaking table tests have been conducted for improved model grounds with various improvement dimensions. To express the variety of improvement dimensions in field, the model ground was created by dividing it into two types of mesh elements, in which each mesh element was simplified as a liquefiable element (unimproved element) or non-liquefiable element (improved element) respectively. Improvement ratio defined by the volume ratio of improved elements in total elements was set for 0% or 50% and the width of mesh element was 50mm. The acceleration of shaking table was applied by step loading; 100, 200, 300 and 400gal with the sinusoidal wave form of 3Hz. In order to investigate the seismic behavior of the improved ground, pore water pressure transducers and acceleration meters were set in the model ground. The main conclusions obtained from this study are as follows; 1) Both of the ground settlement induced by liquefaction and the acceleration response during seismic loading are greatly affected by the generated excess pore water pressure depending on the improvement dimension. Therefore, the "liquefiable region" in which the excess pore water pressure ratio is more than 0.75 is newly defined to evaluate the effect of improvement dimension on the acceleration response of ground, excess pore water pressure and vertical settlement, 2) As improved element increases in the liquefiable region, both of the excess pore water pressure in liquefiable element and the acceleration response of ground surface decrease. Namely, the improvement ratio in the liquefiable region could be an important index to evaluate the effect of improvement, 3) From the experimental conditions in this paper, it is suggested mat 300mm is the best vertical interval and 150mm is the worst one to reduce the ground settlement induced by liquefaction and the vibration of ground.