For the purpose of making clear the limited moisture content to maintain the quality of green tea and the relative humidity (partial vapor pressure) in a packing container which are the important factors to presume the damp-proofing property of packing material which is necessary to maintain the quality during the circulation period of a packaged green tea, the methods of measurement thereof were studied and the following results were obtained;1. It was proved that the limited moisture content to maintain the quality of green tea is 6.5, 6.0, 5.0 and 4.5% each at a temperature of 10, 20, 27 and 35°C during 30 days storage.2. Hygroscopic isothermal characteristics at a temperature of 27+1°C and green tea moisture content of about 3.9% were obtained, from which it was made clear that the relative humidity (partial vapor pressure) developed by green tea with moisture content of 3.0-5.O% within a packing container ranged from about 20 to 40/% R.H.3. At a temperature of 25±2°C, relative humidity (partial vapor pressure) produced by green tea different each other in moisture content was measured with a hygrometer and a high degree (r=0.985) of correlation was observed among them, leading to a regression formula of y=8.55x-3.18. The values calculated there-from were compared with the values obtained from the hygroscopic isothermal characteristics and they were found to be almost coincident with each other within the range of green tea moisture content from 3.0 to 5.5%, for which. reason the said regression formula is seemed to be fully applicable.