Abstract Low cycle fatigue behaviour at room temperature of two 14Cr ferritic steels with ultrafine grains and strengthened by fine oxide dispersion is studied. The two steels differ mainly in the grain size distribution (uniform and bimodal). It is shown that the bimodal grain size distribution has a detrimental effect on the fatigue life time. The rough surface relief is formed in grains bigger than 1 μm in diameter. Nucleation and early growth of fatigue cracks is facilitated by the surface relief which leads to important reduction of the fatigue life.