Invasive species is a major factor that threatens biodiversity and alters global ecosystem. While Lythrum salicaria, a Korean native wetlands plant, is a well-known aggressive invader of wetlands in North America, it is not much spread. Through experiments on competition among the three species (Lythrum salicaria, Phragmites australis and Panicum dichotomiflorum), we explored the mechanism of plant invasion in temperature elevation. The experiment was conducted under different conditions of flooding and temperature which are expected to change in the future. Three species were investigated separately and planted together to watch the competitive effect. There was no difference between wet treatment and 10+ flooding treatment, but plants of dry treatment didn’t grow up well than other water treatments. In competition experiments, plants on dry treatment didn’t show difference between individual and competition but there was little difference in wet and 10+ flooding treatment. It didn’t have an effect what kind of species they were planted together in competition experiments.