The purpose of this study is to evaluate the difference of the thermal environment at the outdoor balcony, such as mansions and so on, with or without the green curtain, which could be expected to come into wide as a measure of the heat island phenomenon in the near future. The actual experiment was carried out for the two classrooms almost as same as the direction, floor and area, and we analyzed measurement data, selecting three days of “Manatsubi” in August. As a result, the velocity of the wind reduced 40％ behind the green curtain, compared with in front of it, and both values collated strongly. The value of MRT also was reduced about 30℃ (40 %) with green curtain, compared with no curtain. The value of WBGT reduced 0.7～1.3℃ at the peak in the daytime with green curtain, compared with no curtain, but the former showed tendency to be 0.7～0.8℃ more over than the later from midnight to early morning. It was suggested that using the thermal indices of MRT and WBGT, we could evaluate the improving effect of the outdoor thermal environment behind the green curtain.