This paper describes the instanteneous cross section void fraction measurement in a double cylinder natural circulation test facility using a wire-mesh sensor built in the vessel cross sections. The inner and outer cylinders with their inner diameters of 0.49m and 0.39m, respectively, were placed co-centrically to simulate basic natural circulation flow to be developed around a chimney of a natual circulation nuclear reactor for next generation. Air was uniformly introduced from the bottom plate of the inner cylinder to drive upward flow in the chimney and downward flow in the downcomer. The natural circulation velocity of water remained almost unchanged while air flow rate was increased. This is mainly due to increased amount of bubbles trapped in the downward flow in the downcomer region.