A preliminary survey of Orchids diversity was carried out in limestone rainforest along Nah-Ha Mountain ridge (about 300 mean sea level). The vegetation of this mountain was dominated by Pondonus monotheca Martelli and Meionorrhoeo sp. About ten species of orchid were recorded from 1 km transect. Among these, 3 species were terrestrial and 7 species were epiphytic orchids. Five species were identified, i.e. Dendrobium sp.l (Auang takap), Dendrobium sp.2 (Auang kangpla), Pophiopedeium godefroyae var. ieucochiium, Cymbidium sp., and Butbophyttum sp. The other five species could not be identified to certain species. Dendrobium sp.l (Auang takap) had the highest relative density followed by Dendrobium sp.2 (Auang kangpla) and Pophiopedeium godefroyae var. ieucochiium respectively. Result from microhabitat analysis revealed that the number of Pophiopedeium godefroyae var. ieucochiium plants found on bare rock were a significantly lower (p<0.01) than those found on litter. Considering the other five epiphytic species, the number of orchids occurred on tree trunks were significantly higher than found on branches. Therefore, low humidity of bare rock and wind exposure of tree branches may responsible for the lower number of orchids.