Abstract: The relationship of OLR and the development of tropical cyclones over the South China Sea is studied by using the OLR data (1990—2000). The results show that there is a good relationship between the development of South China Sea tropical depression and the OLR variation over the South China Sea. Tropical depression would easily develop into tropical storm when OLR low center embedded in ITCZ. Under the state of binary typhoons, the development of South China Sea tropical depression depended on the strength and distances of two OLR low centers. IOD (Index of development) was defined to describe the relationship between the variation of OLR and the development of South China Sea tropical depression quantitatively. Tropical depression would easily develop into tropical storm when IOD≥9.