The salinity gradient power is the energy of the entropy of mixing the two solutions at different salt concentrations. The estuary of the river as a place of the mixture of seawater and freshwater has great potential of this kind of renewable energy. In this research, this potential in Arvand River estuary and its effective factors is analyzed and evaluated. By examining the calculations on the data of three hydrometric stations located on Arvand River, in Khorramshahr station, the mixture of 1.26 m3 of seawater with salinity of 40 ppt with 0.174 m3of river water with 1.17 ppt of salinity leads to the highest energy at 739.85 MW. Also, the penetration length of saline water and water residence time on the estuaries of the rivers leading to the Persian Gulf has a significant effect on the salinity gradient power. According to the calculations carried out under HWS and Arvand River mean discharge, the maximum saline water penetration length into estuary associated with Arvand River is 41 km. The maximum residence time of seawater in Arvand River estuary is 8.9 days.