In a very hot climate equivalent to a Japanese summer, the reduction of heat coming into rooms is very important with respect to thermal comfort and energy efficiency. The objective of this study is to investigate the evaporative cooling effect from roof lawn gardens planted in non-woven fabric as one mode of passive cooling. It was confirmed by field measurements during the summer that the amount of heat coming into the rooms was reduced by a roof lawn garden. That is, the surface temperature of the roof slab decreased from about 60 to 30 degreesC during day time, which was estimated to be followed by a 50% reduction in heat flux into the room by simple calculation. The evaporative cooling effect from roof lawn gardens is considered to play an important role in reducing heat flux. In order to evaluate the evaporative cooling effect of a roof lawn garden, analysis of the heat and moisture transport in the lawn garden is required. Thus, along with those field measurement, a wind tunnel experiment in a room was carried out in order to obtain the basic information for understanding and predicting the heat and moisture transport in the lawn. Furthermore, a numerical calculation by a simultaneous transport model of heat and moisture was carried out using the results of the wind tunnel experiment. The calculated results were in fairly good agreement with the measured values, and the evaporative cooling effect by the roof lawn garden was shown. For more accurate and quantitative evaluation and prediction, sensitivity analysis of the transport parameters and the examination of the proposed model including measurements are required.