Vegetation after the 1977-1978 eruptions on Mount Usu, northern Japan, was monitored for 15 years from 1994 to 2008, by using two plot sizes, 2 m by 5 m and 5 m by 5 m. Species richness and diversity annually increased, and was not saturated for 30. Although richness was higher in the large plots, diversity was not different between the two plot sizes because of low evenness. Well-established species were perennial forbs and trees: Polygonum sachalinense and Anaphalis margaritacea decreased in cover, Trifolium repens and Populus maximowiczii increased, and Petasites japonicus unchanged. Two large clonal perennial forbs, P. japonicus and P. sachalinense predominant soon after the eruptions showed higher cover in larger plots. These results suggested that the cover was estimated higher in the large plots due to the vegetative growth. NMDS was conducted by the vegetation data combined with elevation, distance from the caldera rim, litter amount, ground surface movement and year. The measured environmental factors were related more to the vegetation patterns in the small plots, while yearly vegetation changes were explained more in large plots. Scale-dependent factors may explain the differences between them. The enlargement of plot sizes is appropriate to detect vegetation dynamics when needed, although the careful interpretations are required to investigate fine-scale differences.