Fatigue analysis is an important part of the design process for offshore wind turbines (OWTs). The aerodynamic wind forces are the main fatigue loads, and are generally transformed into six load components: Fx, Fy, Fz, Mx, My, and Mz, which may lead to hotspots of concentrated multiaxial stress. In addition, the six wind load components may be correlated, which makes fatigue analysis complex. To address these issues, this paper derives and presents two new formulae to account for load correlation in the determination of stress power spectral density (PSD) from multiple random loads based on the interaction equation approach and the first principle stress approach. These two formulae form two frequency domain fatigue criteria to evaluate fatigue life of OWT support structures. Two frequency domain criteria are validated through comparison with full time domain analysis results. (C) 2021 Elsevier Ltd. All rights reserved.