Reactivity of void in the channels between the fuel plates is measured, and the impact of core conversion from using highly enriched uranium (HEU) to medium-enriched uranium (MEU) in the light-water-moderated cylindrical core with heavy water reflector is investigated on this quantity at the Kyoto University Critical Assembly. The void was generated in the flow channels by producing nitrogen gas bubbles through a small needle-like nozzle and the reactivity effect was measured. The void fraction was measured in an outof-pile experiment. The results indicate that the void effect on reactivity is slightly larger (more negative) in the MEU core than in the HEU core. It is also shown that the interference effect of reactivity by bubbling in two adjacent channels simultaneously is within the experimental error.