The purpose of this study is to fiure out the floral changes during three years after construction of the Youido Ecology Park. Yearly change of plant species composition, distributed at the research field, had been investigated for three years since 1996. To compare and investigate the changes of the establishment of the Ecology Park, we have analyzed the distribution characteristics of plant species, exotic naturalized species, and so on. The results obtained through this investigation are as follows; 1) Total numbers of plant species were 106, 170, 175, and 148, in the year of 1996, 1998 1999, and 2000, respectively. While the most of cultivated species before th establishment were decreased, the naturalized species such Phargmites communis Trin, and Artemisia selengensis Turcz. etc, were increased. 2) Forty three species of exotic naturalized species were investigated and were increased at the early stage of the establishment, but showed decreasing trend recently. 3) Sixty five species of introduced and planted species were investigated. Most of them were occurred at the first year after the establishment, but since 1999, there has been a significant decrease. 4) Sixty seven species among total 235 were verified as the wetland-adapted plants. The ratio to the total numbers were low, but the occupied area of a few species was expanded. Therefore, it indicates that monitoring on the physical environments of the Youido Ecology Park was urgently required.