AbstractThreshold condition and rate of fatigue crack growth in both short and long crack regime appear to be significantly affected by the degree of crack deflection. In the present paper, a theoretical model of a periodically kinked crack is proposed to describe the influence of the crack deflection degree on the fatigue behaviour. The kinking of the crack is due to a periodic self‐balanced microstress field having length scale d. By correlating the parameter d with a characteristic material length (e.g. average grain size in metals, maximum aggregate size in concrete), the present model is applied to interpret some experimental findings related to crack size effects in fatigue of materials. Well‐known experimental results concerning three different situations (fatigue threshold, fatigue short crack growth and fatigue crack growth in the Paris regime) are analysed.