The long-crested wave properties have been widely implemented in the design of offshore structures. However, long-crested waves are seldom found in the real sea condition. Many research papers have also stated that wave forces obtained by the waves would be overestimated. Indeed, the real sea conditions are better represented by the short-crested waves. Hence, in order to obtain an optimum design for the offshore structures with cost and time effective, consideration of the short-crested nature of waves is necessary. In this paper, a study comparing the responses of the classic spar due to short-crested waves and long-crested waves obtained experimentally has been performed. The model tests have been carried out in the wave tank of Offshore Laboratory for Universiti Teknologi PETRONAS. In the model tests, short-crested waves and long-crested waves were generated. Responses of the classic spar model were recorded by the Optical Tracking System (OptiTrack). The responses of the classic spar due to short-crested waves and long-crested waves were compared among. From the comparisons, the responses for short-crested waves in all the three degree of freedom (DOF) were found to be about 35 less than the responses for long-crested waves. Thus, it could be concluded that optimized and economical designs can be arrived at by considering the short-crestedness of the waves for classic spar structures. © (2014) Trans Tech Publications, Switzerland.