Abstract: An investigation aiming to demonstrate plant species / area of the primary tropical rain forest of south Yunnan has been done. Species diversity , species/ individual relationship and frequency of tree species were discussed. From the analysis of species/ area , the optimal plot size for phytocoenological survey on the forest is suggested to be 0. 25 hm² , which is close to the phytocoenological minimum area of the forest , and the total sampling area for a forest type in which the intrinsic floristic composition can be embodied is also suggested to be at least one hm² (four such plots of 0. 25 hm² with cumulative area one hm²) . It is also concluded that the rain forest formations on non-limestone have higher species diversity than limestone rain forest formations , and the formations on ravine habitats have higher species diversity than the ones on lower hill slopes. The comparison between the rain forest of Yunnan and the ones of the world revealed that the rain forest of Yunnan has a slightly lower species richness than typical lowland rain forests of southeast Asia , but higher than rain forests of tropical Africa.