Rwanda is an East African country where rainfed agriculture is being carried out in more than 95 per cent area. Availability of land for cultivation is 52 percent of total geographical area posing land scarcity for agriculture and need to improve land management and productivity. Presently, less than 3 per cent of the arable land is under irrigation and remaining is under rain fed giving low production due to erratic and poorly distributed rainfall. Rubirizi is somehow moderate rainfall area and distribution of rainfall is insufficient and the soil becomes dry very fast particularly on terraces constructed on laterite hill land. Crop production is affected due to soil moisture stress during dry seasons. Field experiments were conducted in ISAE farm at Rubirizi, Rwanda with an objective to find out effect of supplemental irrigation on maize during May to September 2008. Based on results of the experiment, it can be concluded that supplemental irrigation ensures crop yield and increases land productivity. Climatic data and soil moisture in the experimental fields during study period show that from May to harvesting period in mid September there is insufficient rainfall to meet crop water demand and thus soil moisture content continue to be decreased. Effect of limited water on maize grain yield of short duration variety is considerable and careful control of frequency and depth of irrigation is required to optimize yields under conditions of water shortage. Total crop water demand of maize during the crop season is 405 mm and effective rainfall is 125mm inferring the need of supplemental irrigation for successful crop. From the flowering stage of maize if supplemental irrigation is continued at the rate of 5.3 mm every day and 7.9 mm once in 2 days, dry maize yield of 1.8 Tons/Ha and 1.46 Tons/Ha respectively can be achieved. And if no supplemental irrigation is given from flowering stage it can yield only the fodder. More variations in quantity of supplemental irrigation need to be tested to get real picture of yield reduction against irrigation. Le Rwanda, un pays de l'Afrique de l'est où l´agriculture dépendent de la précipitation, 95pourcent de la surface totale reçoit la précipitation et la surface cultivable est estimé á 52pourcent donc la manque de la surface pour l ́ agriculture ceux-ci conduit au besoin de la gestion du sol et amélioration de la productivité. Aujourd´hui, moins de 3pourcent a bénéficié de l´irrigation et les restes dépendent encore de la précipitation ce qui donnent peu de productivité á cause de l´irrégularité et mal distribution de la pluie. Le sol de RUBIRIZI reçoit une précipitation plus ou moins modéré et leur distribution est insuffisante par conséquent le sol devient sèche très rapidement particulièrement sur les terrasses de colline de la ferme de l´ISAE-RUBIRIZI. La production est affecté par la