This research was focused on identifying the effect of substructures on the ultimate loads of an offshore wind turbine. The ultimate loads of a National Renewable Energy Laboratory (NREL) 5-MW generic model were assessed for two kinds of substructures: the jacket and mono-pile. First, the NREL 5-MW generic model was implemented in the software GH-Bladed(TM) by using data from the published NREL report. The specifications of the mono-pile and jacket structures came from the published NREL and UPWIND reports, respectively. Based on the design load cases and conditions taken from the International Electrotechnical Commission 61400-3 and the published UPWIND project report, we analyzed the ultimate loads of offshore wind turbines to determine the effect of the substructure type. Based on these results, the ultimate loads of mono-pile-type and jacket-type models were compared. As a result, we confirmed the deviation in M-x of the blade, all sections of the tower, F-x of a the hub, and F-x and F-y of the yaw-bearing sections, respectively. Copyright (c) 2015 John Wiley & Sons, Ltd.