The use of glass and plastic greenhouses have rapidly increased in recent years. Meteorological and soil moisture environments inside a greenhouse are significantly different from those in the open field. These differences should be understandable to establish accurate predictions of water consumption and evapotranspiraton inside a greenhouse which play impotrant roles in the design of any irrigation system. This study aims to estimate the micrometeorological conditions inside a watermelon plastic greenhouse which can be used for evapotranspiration calculations. Therefore, micrometeorological data were measured inside and outside the greenhouse for the comparison purposes. The results of this study may be summarized as : (1) Solar radiation inside the greenhouse was about 70% of that of outside it. (2) Net radiation during the daytime inside the greenhouse was almost the same as the absorbed total short-wave radiation. (3) The difference of air temperature inside and outside the greenhouse was very large during the daytime whereas not much difference was observed during the nighttime. (4) Both relative humidity and saturation deficit inside the greenhouse were higher than outside in case of highly vegetative land. (5) Wind speed inside the greenhouse was almost zero, while outside the average value was 1.8m/s.