Prediction of the seakeeping performance of a ship in the earlier ship's design step is necessary to ensure the safety of navigation during sailing. To accommodate such a demand, this research proposes to analyse the seakeeping performance of a rounded hull catamaran. The vertical motion of the catamaran including the heave and pitch motions were quantified by her Response of Amplitude Operator (RAO). The Computational Fluid Dynamic software was used to develop the model, where several effects of various Froude numbers, hull lateral separation ratios and wavelengths on the heave, and pitch motions of the rounded hull catamaran have been taken into account. The results revealed that the higher Froude number associated with less hull lateral separation ratio was proportional to the subsequent increase of her heave and pitch motions. In the case of / ≤1.0, the seakeeping quality of the rounded hull catamaran has been improved and presented in the form of a sufficient reduction of the heave and pitch motions. It is shown that the estimated seakeeping performance of the rounded hull catamaran greatly depends on the Froude numbers and magnitude of wavelength.