Abstract: The determination of representative elemental volume (REV) directly affects the calculating accuracy in permeability calculation for clay using the asymptotic expansion method in geotechnical engineering. Using the sea clay as an example, the influencing factors are studied and compared. By contrasting the results of 4 different types of regular polygon models, and different unit arrangements which are parallel and staggered and different models in 2D and 3D, the calculated permeabilities show large deviations to the measured ones but little differences among each model. It is illustrated that the shape, arrangement and dimensions of the models have little influences in the calculating accuracy. The proposed elliptical particle combined with water film model (E-W model), which takes the consideration of the flattened shape of clay particles and the strong bound water wrapping around them, well improves the accuracy. The representativeness of the physical characteristics of the clay particles is the main influencing factor of REV. The calculated permeabilities of kaolin and illite clay also exhibit high accuracy, which shows that the E-W model can be widely used in the permeability calculation for clay using the asymptotic expansion method.