Abstract:Pot seedlings of Schima superba and Castanopsis fissa were grown under 100 %, 40% and 16% of natural light. The light intensity was cotrolled by using black sieve-cloth. Leaf samples were collected in March,June and September after treatments for one and more than one year. Stable carbon isotope ratio (δ13C value) was determined by a ratio mass-spectrometer, then the intercellular CO2 concentration (Ci) and water-use efficiency (WUE) were calculated from the δ13C values. The decrease of growth irradiance resulted in the rise of both Ci and negative value of δ13C (-‰), and the reduction of WUE. Under 40% and 16 % of irradiance, the δ13C value of Schima us Perba leaves increased - 0. 76‰, and - 0. 89‰, while that in Castanopsis fissa leaves increased - 0. 54‰ and -0. 66‰,respectively. The calculated Ci went up by 11 - 13μl L- 1 in Schima superba and by 8 - 10μl L-1in Castanopsis fissa under low irradiance. Water-use efficiency of seedlings in 100%natural irradiance was about 4μmol CO2 mmol-1 H2O, but it was reduced by 6 - 12% (in 40%light). or 8 - 24 % (in 16 % light). It is concluded that more 12C and less 13C were fixed during photosynthesis of these two woody species under low irradiance. The properties of exchange in water and CO2 were regulated by growth irradiance, and the δ13C value might reflect the incident light status of leaves in the life duration.