Abstract: Obvious differences exist between southern and northern natural conditions of Hainan Island which lies in the tropical area and can be divided into two parts: the southern tropics and the northern tropics. Some scientists also attempted to draw subzone demarcation. Dipterocarpaceae forest, as a sign of tropical forest in Asia, is distributed mainly in southeast of Hainan Island. In addition, plants of Dipterocarpaceae are dominant in tropic evergreen monsoon forest and belong to a zonal vegetation type of Hainan. The authors evaluated the demarcation line on the basis of the distribution of typical forests, Dipterocarpaceae forests, and draw a new line that across from Changjiang through Baisha and Qiongzhong to Wanling. The ecology and geographical distribution of Dipterocarpaceae forests are described.