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Climatic factors controlling plant growth and distribution, and methods of paleoclimatic reconstructions based on plant megafossils are briefly reviewed. Of the two basic methods, the nearest living relative and leaf physiognomy methods, the latter provides more reliable interpretations for older fossil assemblages consisting of many species whose taxonomic affinities are uncertain. As evidenced by leaf physiognomic characters and other geologic informations, overall trends indicated by Japanese Cenozoic floras are changes in temperature under the humid climate. Estimated temperature curves are provided by leaf margin analysis of dicotyledons for these floras. Around the Early/Middle Miocene boundary is the warmest period during the Miocene. The plant megafossil evidence for this warm period indicates a warm-temperate climate : mean annual temperature is less than 20℃ and mean annual range of temperature is nearly the same as or greater than that of the present. The result is inconsistent with that by pollen and molluscan assemblages, and possible explanations for this discrepancy are given.