Stable carbon isotope composition was used as an indicator of the long-term integrated water use efficiency of plants. Because physiological symptoms caused by soil salinity stress develop similarly to those by soil water deficit, it is often considered that the effect of soil salinity on carbon isotope discrimination would be similar. However, several studies found that soil salinity affects carbon isotopic discrimination differently from that of water stress. To investigate temporal changes in δ¹³C with Na/K ratio and soil moisture tension under different soil salinity conditions, lettuce and young radish were grown for 40days under greenhouse condition. Soil EC (soil:water = 1:5) was adjusted by adding a stock solution of KCl, NaCl, CaCl₂, and MgCl₂: 0.2, 0.5, 1.0, and 1.5 dS m^-1. At each measurement EC, two ratios of Na to K (1 and 30) were chosen. During growth, soil moisture tension was adjusted daily to -50±5 kPa at low Na/K and to -35±5 kPa and -50±5 kPa at high Na/K. Dry matter yield of lettuce and young radish was negatively correlated with soil salinity and plant-δ¹³C. In addition, the δ¹³C values became less negative and stomatal resistance increased, indicating decreased carbon isotope discrimination due to decreased CO₂ diffusion as soil salinity increased. An increase in stomatal resistance per unit increase in soil salinity was greater in lettuce than in young radish. In particular, this pattern was further modified by the Na/K ratio. Soil moisture tension was negatively correlated with dry matter yield and δ¹³C for young radish, but this relation was not observed for lettuce. Our results suggest that soil salinity stress decreases carbon isotope composition due to increased stomatal resistance, and that a proportional increase in specific ion such as Na causes further carbon isotope discrimination.