The purpose of this study is to evaluate the difference of the thermal environment at the outdoor balcony with or without the green curtain, which could be expected to come into wide as a measure of the heat island phenomenon. 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, 2013. As a result, the surface temperature of the back side of the green curtain reduced about 4℃, compared with it of the high reflected glass. The difference of surface temperature between with and without the green curtain indicated about 6℃. Because of decrease of the surface temperature, the indoor temperature was also reduced about 2℃ with the green curtain, compared with no curtain. The incident solar radiation reduced more than 80％, and the indoor illuminance reduced more than 30％, at the peak in the daytime with green curtain, compared with no curtain, but the recommended level of illuminance could be maintained in most of daytime.