The catalytic activity of a flexible wire-net photocatalyst for ethylene decomposition was investigated to obtain fundamental infomation for the design of a compact titanium dioxide photocatalytic reactor. The initial activity change was first examined with a newly prepared wire-net photocatalyst: when 50 ppm of ethylene was decomposed by batchwise irradiation with a black light lamp, the time for complete conversion decreased with the increase in the number of cycles of irradiation. It became constant after a few cycles. The photocatalytic activity of 20-mesh wire-net photocatalyst was observed to be nearly equal to that of 350-mesh one under the same amount of titanium dioxide loading of 1.88 g (specific amount 7.2 g/m(2) for 20-mesh wire-net, and 5.1 g/m(2) for 350-mesh wire-net, respectively). The reaction rate of ethylene decomposition was represented by the 0.5 th order of the ethylene concentration. The decomposition rate increased in proportion of the amount of titanium dioxide loaded in the range from 0.92 g (specific amount 3.5 g/m(2)) to 2.70 g (specific amount 10.4 g/m(2)), on a 20-mesh wire -net where 50 ppm of ethylene was treated.