The environment friendly calcareous deposit films were formed on steel plates by electrodeposition technique in natural seawater and synthetic solutions such as dissolved $ extrm{Mg}^{2+}$ and $ extrm{Ca}^{2+}$ ions at various potential conditions. The influence of potential conditions on composition ratio, structure and morphology of the electrodeposited films were investigated by scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS) and X-ray diffractor (XRD). Accordingly, this study was provided a better understanding of the composition between the growth of $ extrm{Mg(OH)}_2$ and that of $ extrm{CaCO}_3$ during the formation of calcareous deposit films on steel substrate under cathodically electrodeposition in synthetic and natural seawater. The results showed that the formation of good overall calcareous deposited film in seawater can be achieved by controlling the Ca/Mg ratio according to interfacial pH with the effective use of the electro deposition technique.