Salinity gradient between two solutions can be transformed to an electrical power by reverse electrodialysis (RED) process which comprises of alternatively cation exchange membrane (CEM), dilute solution, anion exchange membrane (AEM) and concentrate solution. Although spacers are generally equipped between the membranes to keep their intermediate distance, the presence of them also have a significant effect on RED stack resistance, often called as spacer shadow effect. In this study, the spacer shadow effects on both membrane and solution compartment resistances are investigated by using 16 spacers having different geometries with 56-84% porosity and 0.100-0.564mm thickness. In addition, we search and propose the suitable spacer geometric parameter for estimating these spacer shadow effects. The results indicate that the spacer shadow effect on membrane has a good correlation with a parameter consisting of area fraction and diameter of spacer filaments, whereas, a parameter consisting of both area and volume fractions of the spacer agreed well with those on solution compartment in a wide range of spacer geometric condition. These results are useful for not only estimating the spacer shadow effect on both membrane and solution resistances for enhancing the subsequent RED power output performance, but also designing a suitable spacer geometry specified for RED.