Gn. Leuser National Park locates in Aceh and North Sumatra and 10,947 km2 in area. To clarify the vegetation and monitor its future change, we made two 1-ha plots (L1 and L2) at Tangkahan (N3 40’E98 04', 250m in altitude) in the Park in May and November 2014. Two plots were about 200 m distant from each other. L1 was nearer to the boundary of national park than L2 and had some big stumps by illegal logging in the past. The total number of trees (≧　15cm in girth) was 1371 and 1324 trees in L1 and L2 plots, respectively. Basal area was 25.9 m²/ha and 35.6 m²/ha. The biggest DBH cm was 79 and 139 cm, tallest tree was 48m and 54m in L1 and L2, respectively. The family (Engler system) and species numbers were about 50 and 240 in L1, 49 and 310 in L2, and 57 and 440 in total of the two plots, respectively. The most dominant family was Dipterocarpaceae in both plots. About 15 species of Anisoptera, Dipterocarpus, Shorea and Vatica in Dipterocarpaceae composed 17.4% and 14.4% of BA in L1 and L2, respectively. Smaller maximum DBH and less species number in L1 than L2 was probably caused by illegal logging in the past. Only one plot in 17 1-ha plots which we had made on Borneo Island with the same methods exceeded 300 tree species. L2 seemed one of the most diversified forests in Indonesia.