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Fatigue assessment of fixed offshore platforms is essentially important to ensure their structural safety. Because of the small-scale leg diameter, these structures are often drag dominated, and linearized wave force is often utilized for the fatigue estimation. To evaluate the influence of nonlinear effect on fatigue damage, the nonlinear drag force is directly incorporated into the computation of the wave force spectrum based on autocorrelation function. The features and the contributions of nonlinear wave force are first investigated in the frequency domain. Then, structural fatigue damage associated with the linear and nonlinear wave forces is compared using a time-domain fatigue assessment method. Investigation indicates that the influence of nonlinear drag force is closely related to the characteristic diameter of the structure. The nonlinear drag force can contribute up to 10% of the total wave force and lead to about 10% structural fatigue life reduction when the characteristic diameter is small.