With the application upgrade of alloy 718 in the marine oil exploitation, higher corrosion resistance demands has been presented for structure design and reasonable material application of drilling and production equipments; thus there is an urgent need to understand influences of grain size on corrosion behavior of such alloy in sea water environment. In this experiment, different solution treatments are applied to nickel-based alloy 718. As a result, specimens with grain size varied from ASTM No. 9 to No. 3.5 grades. The influences of grain size on the electrochemical behaviors in simulated sea water (3.5% sodium chloride solution) are investigated using electrochemical impedance spectroscopy and polarization curves. Alloy 718 shows good corrosion resistance in this electrolyte solution, however, grain size still has an effect on its electrochemical corrosion behaviors to a certain extent. With the time going, testing results show that coarse-grained alloys with larger grain size have higher impedance values and smaller corrosion current densities, indicating a better corrosion resistance than fine-grained specimens. However, when the grain size increases further, the corrosivity resistant nature of alloy 718 would decline. Testing results show that specimen with size of ASTM No. 5 has the best corrosion resistance.