In this paper,drought,flood and precipitation trend are studied by using month by month precipitation data for last 52 years(1940-1991) at 25 stations in Mongolia.The results show:mean annual precipitation is 216.1 mm,southern and western parts are arid zone,middle and eastern parts are semi-arid zone;annual precipitation variability is 15.6%-38.0%.It is shown that both drought and flood are very frequent in Mongolia.The frequency of drought is higher then the flood,the frequency of severe flood is the two times of the severe drought.Annual precipitation distribution is normal distribution,and it has the quasi-three years and 11-14 years periods.Annual precipitation increases about 0.34 mm/decade years.Annual precipitation increases in the rate of 5.13 mm/year in 1980's and it continue increases in 1990's.