A 3.5 m core taken from Chitsai Lake (altitude 2890 m) of central Taiwan shows the vegetational and environmental changes in a Tsuga forest since 5000 yr BP. During about 5300-4840 yr BP (Zone 4), the Tsuga forest experienced a clear fluctuation toward a Tsuga-Cyclobalanopsis mixed forest. The high percentages of Cyclobalanopsis in the lower half of this zone (Subzone 4b) indicate the last episode of the mid Holocene warm interval. Tsuga forest was well established in the later half of this zone. The extremely abundant spores as well as the finest sediments together indicate high lake level and moist climatic conditions. Between about 4840 and 3730 yr BP (Zone 3), a warmer episode occurred in which Cyclobalanopsis increased slightly compared with Subzone 4a. following this, during about 3730-2030 yr BP (Zone 2), the relatively high percentages of Abies, Ericaceae and Tsuga as well as lower percentages of Alnus and Pinus indicate cooler but stable environmental conditions. Zone 1 represents vegetational changes during the last 2000 years. The increase in Cyclobalanopsis indicates warmer conditions than before. The remarkable decline of Tsuga and Ericaceae accompanying the increase in the secondary forest elements such as Alnus and Pinus may indicate a change in environment. The larger grain size of silt in Zone 1 than in Zone 4, a higher erosion which may be due to strongerprecipitation seasonality in a warmer climate. The upper 15 cm shows rapid increase in secondary forest elements such as Pinus and Alnus. Whether it is related to the increasing atmosphere CO₂ is worthy of further investigation.