•Effects of defects, surface roughness and HIP on the fatigue strength of a Ti6Al4V by AM were studied.•HIP improved fatigue strength to the level of ideal fatigue limit if surface roughness is removed.•Evaluating method for the effective size √areaeffmax of irregularly shaped defects was proposed.•AM fatigue design method was proposed with the √area parameter model and the statistics of extremes. The additive manufacturing (AM) is expected to be a promising manufacturing process for high strength or hard steels such as Ti-6Al-4V for the aerospace industry components having complex shapes. However, disadvantage or challenge of AM is presence of defects which are inevitably contained in the manufacturing process. This paper focuses on the effects of defects, surface roughness and Hot Isostatic Pressing (HIP) process on the fatigue strength of a Ti-6Al-4V manufactured by AM. Defects were mostly gas pores and those made by lack of fusion. Many defects which were formed at subsurface were eliminated by HIP and eventually HIP improved fatigue strength drastically to the level of the ideal fatigue limit to be expected from the hardness. Surface roughness had strong detrimental influence on fatigue strength. The method for estimating the effective size √areaeffmax of irregularly shaped defects and interacting adjacent defects was proposed from the viewpoint of fracture mechanics.