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