Fog is an important supplemental source of water in regions with heavy fog and little rainfall. In the present study, we used the stable isotope content of xylem water to investigate the dry season water source partition between soil and fog water of two evergreen tree species (Cleistanthus sumatranus and Lasiococca comberi), two deciduous tree species (Mayodendron igneum and Lagerstroemia tomentosa), two liana species (Acacia pennata and Combretum latifolium) and understory seedlings of three tree species (C. sumatranus, L. comberi and Celtis philippensis) in a tropical karst forest in southern Yunnan, China. The soil gravimetric water content (GWC) and volumetric water content (VWC), pre-dawn leaf water potential (Ψpre-dawn) and leaf water absorbance were also measured. There were no significant differences in both GWC and VWC among 10 to 100 cm depths in the dry season. In the dry season 2009, Ψpre-dawn of the two evergreen tree species and the understory seedlings down to 4.5 to 6.3 MPa, and which were significantly lower than those of a normal dry season. The proportion that fog contributed to xylem water ranged from 15.8% (understory seedling of C. sumatranus) to 41.3% (liana species C. latifolium). After immersing in water for 3 h, leaves of all the studied species absorbed a significant volume of water. Our results reveal that the woody plants in the tropical karst forest of Xishuangbanna use fog water as an important supplement in the dry season, with more fog water being used by lianas than by trees.