Generally, floating structures play an important role for exploring the oil and gas from the sea. The force and motion prediction of offshore structures may be carried out by using time domain or frequency domain or model tests. In this paper the frequency domain analysis used because it is the simplified and linearized form of the equations of motion. Mostly in numerical calculations the number of meshes plays an important role in the accuracy of results, time of calculation and facing to computer memory limitations. The 3D source distribution panel method is shown to be sensitive to mesh near the resonance frequencies of the floating body. So, it is important to establish best practices and determine the mesh requirements for a given level of accuracy. The results obtained from numerical commercial software HydroSTAR (Hstar) on semi submersible prove it.