Abstract: The sodium void reactivity effect is one of the most important safety issues of sodium-cooled fast reactor (SFR) in nuclear design and safety analysis. In this paper, based on the nodal diffusion method and perturbation theory, a calculation method on sodium void reactivity was established, and the total sodium void reactivity, spatial distribution and physical components for MOX-fueled 1 000 MWe SFR were calculated. The results show that the main contributors to the sodium void reactivity lie in two aspects, the increasing of leakage and the hardening of spectrum, which are opposite both in sign and in spatial distribution, resulting in a strong dependence upon space. For the reference MOX-fueled core, the sodium void reactivity reaches approximately 3 . The results reveal the mechanism of sodium void reactivity and its distribution, and provide reference for the design of low sodium void reactivity cores.