In this research , we investigate the effect of feed flow velocity in both monovalent (Na+) and divalent (Mg2+) salt water solutions to harvest the electrical energy from the reverse electro dialysis (RED) membranes module. The synthetic sea water solution uses 0.5 M concentration of salt and the synthetic river water salt concentration of 0.017 M were used. The carbon electrode was used, due to its low cost, higher melting point, and insoluble in water. Variation of feed flow velocities (sea water and synthetic river water) of 8 cm2/s, 14 cm2/s, 19 cm2/s were used, in addition to compare electrical energy produced from the used of monovalent and divalent ions. The best result was obtained by using the velocity of feed solution 19 cm2/s with the electricty value of 8.033 mV, 0.002 Ω/cm2 and power density of 1,141 mW/m2, while the influence of the addition of Mg2 +ion in the feed solution resulted the electricty value of 4.47 mV, 0.003 Ω/cm2 and power density of 0.15 mW/m2. Albeit the results, some more configuration is needed and worth to be investigated in the future.