The objective of this research was to elucidate the role of sodium ion (Na+) on photosynthetic machinery and growth characteristics in both salt-tolerant (HJ) and salt-sensitive (PT1) rice varieties grown under iso-osmotic salinity stress. The Na+ and Na+/K+ in HJ and PT1 rice seedlings were increased with increasing salt concentrations in the culture media. K+ in salt-stressed HJ seedlings was increased but that in salt-stressed PT1 seedlings was unchanged. The Na+ accumulation in salt-stressed seedlings was negatively related to the water potential in HJ (r2 = 0.78) and PT1 (r2 = 0.72), leading to pigment degradation i.e. chlorophyll a (Chla), chlorophyll b (Chlb) and total carotenoids (Cx+c). The water potential in both HJ and PT1 salt-stressed seedlings was positively related to the Chla concentration (r2 = 0.78 and r2 = 0.58). Furthermore, Chla and total chlorophyll (TC) concentrations in HJ and PT1 salt-stressed seedlings were positively related to water oxidation in PSII (PSII) as maximum quantum yield of PSII (Fv/Fm) (r2 = 0.73 and r2 = 0.68) and quantum efficiency of PSII (ΦPSII) (r2 = 0.92 and r2 = 0.91), respectively. The ΦPSII in HJ and PT1 salt-stressed seedlings was positively related to the dry matter (r2 = 0.86 and r2 = 0.67). The K+ accumulation in HJ salt-stressed seedlings may play a major role in salt defense mechanisms, leading to enhance on photosynthesis capacity, water oxidation in photosystem II (PSII) and growth abilities.