•Fe-1.7C-11.3Cr-1.9Ni-1.2Mo roller steel is tested to a thermal fatigue.•Surface layer degradation is characterised for various fatigue conditions.•Microstructure prior and after thermal fatigue is characterised.•Quantitative assessment of thermal fatigue resistance of roller steel is presented. Thermal fatigue resistance of a high Cr hot work roller steel is evaluated at 500, 600 and 700 °C. Surface layer degradation was investigated and characterised after a finite number of thermal cycles. Degradation mechanisms of the cooled surface layer in relation to the size, shape, orientation, distribution of carbides and crack oxidation progress were elucidated. Complex phenomena of crack growth is discussed where internal cracks nucleate due to stress at crack tip and oxidation along carbides follows from crack tail. Quantitative evaluation of cracks shows increased crack length with increasing temperature and number of thermal cycles.