This study focuses on the low cycle fatigue mechanism of the quenched 9Ni martensitic steel containing retainedaustenite films at martensite laths. HRTEM and EBSD suggested that extrusions developed along Low AngleBoundaries or along High Angle Boundaries at low and high strain respectively. Short cracks grew along LAB atlow strain while they used any kind of boundary as well as crossed the martensite laths at high strain. Retainedaustenite allowed a flowing of matter along interfaces by acting as a lubricant. Consequently, the Coffin-Mansonrelation exhibited a bilinearity with a change in curve slope at Δεt = 1%.