Atmospheric pressure plasma jets have recently received significant attention due to their unique capabilities and novel applications. Various chemical species, such as NO 2 , HNO 3 , O 3 , and OH, generated in air by plasma are dissolved and transported in water quickly, and accompanied chemical stimuli can inactivate bacteria. In our previous study, focusing on OH and H 2 O 2 production, we developed a mist-plasma jet (MPJ) generated using dry helium gas mixed with pure water mist as an alternative to the traditional method using only dry helium gas, known as the dry-plasma jet (DPJ). In this study, the observation and comparison of effects of both MPJ and DPJ on HeLa cells surrounded by cell culture medium immediately after irradiation by plasma and following 24 h were focused. In addition, to observe details of effects of both plasma jets on HeLa cells, two experimental procedures were prepared. One is that of irradiated culture medium, including that with cells and observed cells replaced with fresh cell culture medium following 24 h. The other is that of irradiated culture medium, without cells, with plasma, and observed cells exposed to the plasma-treated culture medium after 24 h. These experiments revealed that MPJ more greatly influences cell death than DPJ.