Abstract: Structures with multiple columns are widely used in ocean engineering, and wave loads on large-diameter columns in a group are quite different from the isolated ones. In the present study, a three dimensional hydrodynamic code DIFFRACT based on potential flow theory is used to calculate wave loads on a single circular cylinder and twin circular cylinders. The accuracies of numerical results are verified by comparing with the corresponding analytical solutions. The wave loads on multiple circular columns in regular waves have been further investigated. Multiple peaks have been found in the results due to the hydrodynamic interactions between columns, which can reach 2.30 times of wave loads on isolated column. The locations of columns with maximum loads are affected by the layout of the group.