During the Plio-Pleistocene boundary (2.7 ~ 2.4Ma), the glaciation of the Northern Hemisphere started. Previous research reported the expansion of northern hemispheric continental ice sheet and the climate deterioration began at around 2.7Ma. On the contrary, other studies argued that 2.7Ma was an interglacial period. Therefore, more evidences are necessary to clarify the debate. Here we use organic carbon isotope (delta~(13)C),leaf size and leaf length/leaf width ratio of fossil Metasequoia needles as the proxy of paleoenvironment to assess if 2.7Ma ago was an interglacial period or not. The fossil Metasequoia needles were collected from five localities in central Japan: Sennan, Hachioji, Higashiomi,Tokamachi and Ikoma. The ages of these materials varies from 3.00Ma to 0.95Ma (Late Pliocene to Pleistocene). For each locality, we prepared 1 ~5 samples for delta~(13)C analy sis, and 5 leaves for leaf size and leaf length/leaf width ratio measurements. These three characters have been shown to be mainly affected by temperature and precipitation of the growth environment. delta~(13)C values are negatively correlated with temperature and precipitation whereas leaf size and leaf length/leaf width ratio are positively correlated with temperature and precipitation. The results of this research show that: the lowest delta~(13)C value (-29.05%o) was measured for the period around 2.6~2.7Ma (Late Pliocene),indicating a warm and humid environment. The leaf area and leaf length/width ratio of fossil Metasequoia needles during 2.6 ~2.7Ma (Late Pliocene) are also larger (7.45mm2 and 4.22 respectively),supporting that 2.6~2.7Ma is a warm and humid period. Thus, our results indicate that there is an interglacial period at the Plio-Pleistocene boundary (around 2.7Ma). After that period, the climate deteriorated, as the environment became cooler and dryer. This climate change is corroborated by the mega fossil records, as the succession of floral assemblages during Pliocene to Pleistocene also indicated a similar trend.