Abstract: Peat formation is considered to be controlled by both water and heat conditions in this paper.A comparison made by the author between present peat distribution and the climatic record of 841 stations in the world during the years of 1961-1970 provides the climatic condition of peat formation.It is well explained by the following equation:Z=5.7K+12-T T(mean air temperature of days with >10℃) respresentes the thermal condition of those days, it can be evaluated from palynological information.Z(water-heat index of peat formation) can be acquired by ¹⁴C dating of peat deposits.Therefore,K(ratio of precipitation to evapotranspiration) can be obtained from T and Z.The humid and arid fluctuation curves are figured at Kyoto,Tokyo and Sapporo in Japan, and Beijing, Changchun and Zhenjiang in the eastern part of China.The humid and arid fluctuation of East Asia in the last 10000 years on peat foma-tion are summarized as follows:1)ca.10000-8500 a.B.P.Arid climate is prevalent especially in the northern part of Esat Asia.2)ca.6000-5000 a.B.P.Humid climate accompanied by the warm period“Hyp-sithermal”is generally predominant.The beginning of humid climate is, however, delayed ca.2000 years and the humid period are not coincide with the warm period in the northern part of East Asia.3)After ca.3000 a.B.P.Increased aridity are recognized in the eastern part of China.