Crystal structure determination has been accomplished for (K-0.3(+), Cs-0.7(+))-beta-ferrite with beta-alumina structure to investigate the mixed alkali effect in ionic conduction. According to the structure of conduction plane, BR sites were occupied by cesium ions and mO sites were occupied by potassium ions. It was concluded that blocking effect of cesium ions in BR sites dominate the mixed alkali effect in this ferrite.