The effects of facet discretisation on the calculated heave added masses for a submerged pontoon are investigated by using the three dimensional calculation method as well as the two dimensional ones. The results show that : When the number of facets approximating the model shape is small, it may cause about 10% difference in the calculated added masses when it is compared with the results obtained by using a large number of facets. Particulary the approximation at the corner part of the pontoon is effective. Therefore, in the estimation of the heave added masses for a semisubmersible offshore structure by use of the singularity distribution method, the number of facets for the lowerhull part should be carefully chosen.