The　influence　of　quartz　tube　size　on　the　surface　distributions　of　reactive　oxygen　species　(ROS)　in　model　tissue　treated　with　a　He　+　O-2　plasma　jet　was　investigated.　Gelatin　gel　was　used　to　construct　the　model　tissue,　and　a　KI-starch　color　agent　was　mixed　with　the　model　tissue　to　visualize　the　distribution　of　ROS.　With　increasing　quartz　tube　diameter,　the　uniformity　of　the　ROS　distribution　on　the　model　tissue　decreased,　and　the　change　in　the　surface　distribution　area　of　the　ROS　on　the　model　tissue　over　the　irradiation　distance　was　quite　different　for　different　quartz　tube　diameters.　The　surface　distribution　of　ROS　on　the　model　tissue　was　affected　mainly　by　the　working　gas　flow;　thus,　the　diffusion　range　of　the　working　gas　flow　on　the　model　tissue　surface　determined　the　surface　distribution　area　of　ROS　on　the　model　tissue.　The　working　gas　flow　was　accelerated,　and　the　diffusion　range　of　the　working　gas　flow　on　the　model　tissue　surface　expanded　when　the　plasma　was　ignited,　resulting　from　the　modification　of　the　working　gas　flow　by　the　electrohydrodynamic　(EHD)　effect.　The　EHD　effect　on　the　expansion　of　the　diffusion　range　of　working　gas　flow　on　the　model　tissue　was　different　for　different　quartz　tube　diameters,　and　the　effect　was　determined　mainly　by　the　discharge　current　density　of　the　plasma　jet　and　the　plasma　propagation　length.