The habitat of threatened damselfly, Mortonagrion hirosei, is reed communities where they stayed throughout their lifespan, from egg to adult. In 1998, one of the local populations was discovered in Mie Prefecture and the habitat was considered to be too small (500m²) to maintain the population after surroundings are developed. Thus, the new reed community (2100m ²) was artificially established adjacent to the natural habitat by transplanting reed rhizomes. Because both sexes have low flight activity throughout their life resulting in low dispersion ability, a few adults immigrated to the new reed community in the first year of the establishment. The distribution of their offspring in the established habitat was gradually extended year by year, during when the reed community had grown up as well as original habitat. The daily number of adults from May to August was estimated using the line transect method which was comparable with the mark-and-recapture method. The estimated population size in the new habitat had increased and reached the similar density with that in the natural habitat 4 years after established. The population dynamic model was proposed based on the yearly estimated population, indicating that the population fluctuation in the new habitat will be stable after 17 generations.