Yemen Arab Republic is located in the south-western corner of the Arabian Peninsula. The area lies between parallels of latitude 12°30' and 17°30'N. and meridians of longitude 42°30' and 46°30T. Yemen was known as “Arabia Felix” and had high civillization in ancient age. But since its hard natural environments, the development of this country fell behind the time. Among of them, water supply problem is the most important matter. In response to the request made by the Government of the Yemen Arab Republic, the preliminary survey of the “Rural Water Supply Projects were carried out by the Japan International Cooperation Agency (JICA). In this paper at first, we elucidate the background of water supply problems and the feature of geohydrological condition of this country. Next, we clarify the mode of occurrence and movement of groundwater which is restricted by the condition mentioned above.<BR>The summary of this paper is as follows : <BR>1. Topographical feature.<BR>i. Yemen's main watershed runs from north to south and divides the country into two parts, western half and the eastern half.<BR>ii. The main watershed or central highland is comparatively flat and in some areas tectonic basins are pointed out.<BR>2. Hydrological feature.<BR>i. As shown in Figure 7, the highest annual rainfall falls in the central highlands and the lowest in the eastern lowlands and the western lowlands.<BR>ii. Springs are observed in the wadi beds and support some small streams in the mountain area.<BR>3. Hydrogeological feature.<BR>i. Geological succession is shown under.<BR>(lower to upper) <BR>Basement complex-Pre-Cambrian<BR>Wajid sandstone-Ordovician<BR>Kohlan series-Lower Jurassic<BR>Amran series-Upper Jurassic<BR>Tawilah group and Medj-Zir series--Cretaceous to Paleocene<BR>Yemen volcanics-Upper Cretaceous to Tertiary<BR>Baid formation-Miocene to Pliocene<BR>Quaternary basalts-Alluvium ii. Main aquifer in this country is Alluvium, Quaternary basalts and Yemen volcanics. On the other hand the rest can be regarded as aquitard or aquiclude, (especially basement complex as aquifuge).<BR>iii. As shown in Figure 12, aquifers are exposed in the central highlands and in the eastern and the western lowlands and aquitards or aquicludes are exposed in the mountain area.<BR>4. Mode of occurrence and movement of groundwater.<BR>i. From mentioned above condition, the central highlands can be regarded as recharge area of groundwater.<BR>ii. For the most part, perennial streams in the mountain area are feeded by the outflow of groundwater from the central highlands.<BR>iii. In Tihama plain and the eastern lowlands alluvium underlies a thick cover of eolian deposits and for its high infiltration capacity, surface flow change into groundwater rapidly.