Design optimization of ironless multi-stage axial-flux permanent magnet generators for offshore wind turbines Direct-driven ironless-stator machines have been reported to have low requirement to the strength of the supporting structures. This feature is attractive for offshore wind turbines where lightweight generators are preferred. However, to produce sufficient torque, ironless generators are normally designed at large diameters, which can be a challenge to the machine structural reliability. Ironless Multi-Stage Axial-Flux Permanent Magnet Generator (MS-AFPMG) embraces the advantages of ironless machines but has relatively small diameter. The objective of this article is to present the design optimization and performance investigation of the ironless MS-AFPMG. An existing design strategy, which employs 2D and 3D static finite element analyses and genetic algorithm for machine optimization, is improved with the aim to reduce the calculation load and calculation time. This improved design strategy is used to investigate the optimal ironless MS-AFPMG. Some intrinsic features of this kind of machine are revealed.