Orchid mycorrhiza is known to be beneficial to seed germination and growth of orchid. In this study, twenty seven isolates of Rhizoctonia-like fungi were isolated from roots of three economically important orchid genera Paphiopedilum, Cymbidium and Dendrobium. The roots were obtained from mature orchids collected from diverse horticultural settings in Chiang Mai and Chiang Rai provinces. Fungal identification was done by the morphological characterization, the comparison of the internal transcribed spacer (ITS) and 5.8S ribosomal DNA (rDNA) sequences, and the phylogenetic analysis. Epulorhiza repens was the most common species found in the roots of various species of all three orchid genera, whereas, Epulorhiza calendulina-like isolates were strictly found in the roots of Paphiopedilum species. Other fungal isolates obtained were identified as an anamorph of Tulasnella irregularis, four new anamorphic species in the genus Tulasnella, and a new anamorphic species in the family Tulasnellaceae. Eight of the 27 fungal isolates were tested for their effectiveness in promoting in vitro seed germination and protocorm development of Grammatophyllum speciosum and Dendrobium draconis, native Thai orchids. The developmental stages of the seeds and protocorms cultured on Murashige and Skoog medium (MS), oat meal agar (OMA), or the OMA inoculated with one of the eight fungal isolates were evaluated weekly. Two isolates of E. repens, Da-KP-0-1 and Pv-PC-1-1, were found to be the most effective fungi in promoting the protocorm development of the G. speciosum. At week 13, the protocorms co-cultured with either one of these two fungal isolates, on the average, were significantly more advanced than those sown on the OMA. The protocorms co-cultured with the isolate PV-PC-1-1 was also significantly more advanced than those cultured on the MS medium. For the D. draconis seed germination, three fungal isolates of different anamorphic species of Tulasnella, C1-DT-TC-1, Pv-PC-1-1, and C3-DT-TC-2, were found to be the most effective fungi in promoting the protocorm development. However, none of these fungal isolates outperformed the MS medium. Our study provides information on diversity of root-associated fungi of the highly traded orchid genera at the sampling sites that were rarely addressed in the previous studies and the potential use of these fungi for the cultivation of orchids.