Abstract: >We studied the pteridophyte flora of Diaoluo Mountain in SE Hainan. In total, 213 species and 4 varieties of 100 genera in 43 families of pteridophytes were recognized. The two dominant families of the flora were Polypodiaceae with 12 genera and 24 species, and Hymenophyllaceae with 11 genera and 17 species. Five species collected recently from Diaoluo Mountain, i.e. Lygodium subareolatum, Pteris longipinnula, Pteris longipes, Allantodia deoderleinii and Dryopteris fuscipes, are new records to Hainan Island. Diaoluo Mountain is one of the centres of pteridophyte diversity in Hainan with a high species density of 55.8 species per centum square kilometer. Its pteridophyte flora is characterized by tropical distribution elements, represented by 100% tropical families, 93.2% tropical genera and 95.3% tropical species (excluding cosmopolitan taxa). Its flora is closely related with that of Indo China and that of Guangdong and Guangxi Provinces of China. The altitudinal distribution of pteridophyte can be divided into three zones: 30% of the pteridophytes are distributed in tropical rain forests and open area below 600 m, while 60.6% are mainly distributed in montane rain forests between 600～1000 m, and 9.4% are distributed in montane evergreen short forests above 1000 m. Most of the pteridophytes below 600 m are heliophytes, while those between 600～1000 m are sciophytes, and those above 1000 m are mainly epiphytes or lithophytes. According to 2001 IUCN Red List Categories and Criteria, 36 species are threatened on Hainan Island. Among them, six species are CR (Critically Endangered), 13 species are EN (Endangered) and 17 species are VU (Vulnerable). Eight of the 36 species are second class protected wild plants in China. The threatened species are mainly caused by  drying of habitats. Because the primary forests had been destroyed on a large scale throughout the whole island, the water retaining capacity of the vegetation and soil has been decreasing for decades. In addition, human disturbance also accounts for some threatened species.