In the case of a hypothetical failure of a residual heat removal (RHR) systems under mid-loop operation, vapor generated in a reactor core forms two-phase flow in a stagnant liquid and rises the water level in the core. The vapor flows into a steam generator through a hot leg, and condenses in the steam generator. Since the flow rate of vapor from the reactor core to the hot leg depends on the water level and the void fraction α in the reactor core, the reliable analysis of the RHR failure cannot be carried out without accurately estimating the void fraction in the reactor core. Although a number of studies on void fractions in two-phase flows in rod bundles have been carried out, there are few experimental data on void fractions in rod bundles under the stagnant condition. Void fractions in four by four rod bundles under the stagnant condition were measured for a wide range of gas volume fluxes to examine the validity of available void correlations. Flow patterns were visualized by using a high-speed video camera to examine the effects of flow pattern on the void fraction. As a result, the following conclusions were obtained: (1) Dependence of the void fraction on the gas volume flux J G changed at J G 1.5 m/s due to the flow pattern transition. (2) Murase's correlation agreed well with the void fraction in the two kinds of rod bundles having different dimensions under the stagnant condition.