Pollen analysis of a 15.0 m core covering the past 3500 years from the Ilan Plain, northeastern Taiwan, displays a record of environmental changes and climate trends for the late Holocene. At about 3500 yr B.P., brackish water-tolerant elements, such as Bruguiera and Acrostichum, dominated the vegetation, reflecting that the environment was influenced by marine excursions. Following this period, wetland elements Poaceae and Cyperaceae were abundant between ca. 3350 and ca. 3100 yr B.P. Then pioneer taxon Alnus increased at the expense of those herbs in the later part of the interval ca. 3100 and ca. 2000 yr B.P. This suggests a shift from a primarily coastal (ca. 3500 yr B.P.) to wetland (ca. 3350 yr B.P.) and then finally terrestrial (ca. 2000 yr B.P.) environment at the coring site. Since then, the present fluvial plain has existed. Humidity conditions which were interpreted from the amount and variations of fern spore probably indicate a strengthening of the monsoon at ca. 3200, ca. 2000, and ca. 900 yr B.P., respectively. The Medieval Warm Period is also apparent indicated by the amount of arboreal elements based on pollen concentrations ca. 1300 to ca. 900 yr B.P. Significant human-induced vegetation changes occurred between ca. 1300 and ca. 900 yr B.P. and over the last 400 years.