Piles are mainly used to transfer vertical  loads  in soil ground. They are also frequently employed in port engineering to bear lateral thrust and to resist lateral deformation in the soil ground. As the horizontal load undertaken by a lateral bearing pile is not only through its shaft friction but also through the pressure difference between the retaining landside and its opposite seaside, the concerning mechanism is so complicated that so far there is no sophisticated procedures to exactly calculate the lateral action. In the current design practices of sheetpile bulkhead, lateral loading pile is simplified as a retaining wall with an equivalent breadth assumed to be equal to twice the pile’s width. The validity of such empirical method, however, is widely questioned; hence an investigation is needed to verify its effectiveness. An investigation by means of centrifuge model testing is carried out to determine the influence of ratio of space to width on equivalent breadth of one pile. Miniature earth pressure cells are installed on both sides of the pile to measure the distribution of pressures. Close correlation is found between the equivalent breadth of one pile and the ratio of space to width: the equivalent breadth ratio keq is approximately linearly increased with the ratio of space to width d/b as long as the value of d/b is less than 4; when the value of d/b exceeds 4, the value of keq is progressively proceeding to a constant, without significant change. In the common case of d/b=2, the observed value of keq is less than 2.0.