There are two components of variation in carbon isotope composition (expressed as the ratio of stable carbon isotope relative to PDB, denoted δ13C) of plant tissues under salinity conditions: one is the effect of physiological processes; the other is the high salinity-induced switch from C3-photosynthesis to Crassulacean acid metabolism or C4-photosynthesis. The former is a main factor affecting plant δ13C values, whereas the later generally has no significant effect on whole tissue δ13C. The relationship between δ13C and salinity is relevant to intrinsic salt-tolerance, salinity level and the period when plants grow under saline conditions. Non-halophyte and halophytes have different response patterns in δ13C to salinity in terms of the mechanism of carbon isotope discrimination. For non-halophytes, δ13C values will increase with increasing salinity when the stomatal closure is the major factor in restricting photosynthesis. As non-stomatal limitation becomes the key factor with the stress intensified, however, the values of plant δ13C may decline. It should be noted that the correlationship between δ13C and salinity is not exhibited by mild stress treatment. For halophytes, the lowest δ13C occurs at a favourable salinity, and the values increase when the salinity is lower or higher than the optimum level. In dense woodlands, the CO2 derived from respiration recaptured by leaves using photosynthesis may alter their plant’s δ13C due to the different δ13C from that of the air. So, further investigations are necessary covering a large range of salinity and duration of treatment, to explore the response of plant δ13C to salinity, and the salt-tolerance of species.