Abstract: Unprotected loss of flow accident is severe for sodium-cooled fast reactor in that it will cause the boiling of sodium and then cause severe accident. For large MOX fuel sodium-cooled fast reactors, the sodium void reactivity is positive, which can make the consequences of unprotected loss of flow accidents more severe. The design of the upper sodium plenum is an effective method for reducing sodium void reactivity in MOX fuel. The purpose of this paper is to evaluate the effect of upper sodium plenum design on the unprotected loss of flow accident in the large MOX fuel 1000 MWe sodium-cooled fast reactor. Based on multi-group nodule diffusion method and perturbation theory, the total sodium void reactivity, spatial distribution and physical components of MOX fuel core with upper sodium plenum in 1 000 MWe sodium-cooled fast reactor were calculated. Based on the results of the calculation of sodium void reactivity, the transient analysis of the 1 000 MWe sodium-cooled fast reactor core was carried out by using the transient analysis code CODA developed by China Institute of Atomic Energy. The assessment of the CODA code was carried out, by using the code to model experiments that simulate a loss of flow accident in a sodium-cooled fast reactor. Comparison of the predictions with experiment, confirm the ability of the CODA code to predict the principal sodium boiling phenomena. In order to better evaluate the reactivity effects, especially the sodium void reactivity, the CODA code adopted a model that considered the spatial distribution of reactivity feedback. The unprotected loss of flow transients with and without sodium plenum were evaluated respectively by CODA code. For the traditional core without upper sodium plenum, sodium void reactivity is strongly positive. In the case of an unprotected loss of flow accident, rapid generation of sodium vapor in the central part of the core leads to introduction of positive reactivity, rapid increase of power, and finally leads to fuel melting and relocation. For sodium-cooled fast reactor designs with large upper sodium plenum, sodium void reactivity is negative in the upper part of the core. Sodium vapor generated in the unprotected loss of flow accident could be limited to the upper part of the core which leads to introduction of negative reactivity and decrease of power. The analysis results show that new design features of upper sodium plenum design are able to significantly improve this unprotected loss of flow accident behavior.