Abstract: The vadose water within the surface layer with depth of about 2 m of 7 megadunes and 1 movable dune was observed during camel expeditions of 1997-2003.The relative height of these megadunes ranges from 300 to 400 m with the altitude of 1 300 m^-1 488 m a.s.l.Traditional methods are used to probe the vadose water content and the physical features of its eolian deposit.Observations were made during 25-27 September every year at different spots in the hinterland of that sand sea.The results show that for all of the observed megadunes,the vadose water content appears to increase volumetrically with the depth with an amount of about 3% at the depth round 2 m below the megadune surface.It reaches to about 65% of the maximum wetness held by matrix potential of the deposit (HB) which was measured after rainfall events during the expeditions of 2000 and 2003.The positive δ¹⁸O values of the vadose water at the depths from 25 cm to 1 m beneath the megadune surface reveal a repeated recharge-evaporation process.The infiltration depth of a few centimeters after rainfall event was also observed along a slope of 120 m on a sand dune,it could be quickly exausted through the evaporation measured by energy budget.It is concluded that the local precipitation with annual mean of about 50 mm from its northwest to 80 mm southeastwards seems not sufficient for the output amount of annual evaporation from megadune surface even if a portion of it being condensed.If take into account the high percentage of HB of the vadose water within the surface layer of megadunes and the enrichment process of ¹⁸O,the current concept that the main recharge source of the megadune area is the local precipitation will be challenged.The existence of an important groundwater recharge source far away from this dune desert,perhaps,from the mountainuous area to its south,is not directly identified,but it seems to be implied.