Cu-Co alloy substrate was formed on the Au-seeded p-GaN side of sapphire wafer specimens to function as a secondary substrate of vertical structure GaN-based light emitting diode (LED), using electrodeposition method at constant potentials in an acidic citrate electrolyte. Potentiostatic deposition process was available to change the Co content of Cu-Co alloy substrates with varying cathodic deposition potential. Rotation speed of wafer specimens during electrodeposition also affected seriously the microstructure as well as Co content of the substrates. Cu-Co alloy layers deposited on non-rotating specimens showed coarse, particulate microstructures, whereas Cu-Co alloy substrates formed with applying specimen rotation had dense, grained microstructures. The chemical composition and microstructure of electrodeposited Cu-Co alloy substrates were closely related with their electrical and mechanical properties, which should be essentially considered for a appropriate secondary substrate of vertical structure LEDs. Residual stress, electrical resistivity, and Vickers hardness of the substrates were measured, respectively, according to cathodic deposition potential and rotation speed. In conclusion, mechanically strengthened Cu-Co alloy substrate in comparison with ductile Cu substrate was fabricated having very low residual stress and acceptable electrical resistivity by alloying Cu with low Co content of less than 3.15 at% in potentiostatic deposition process. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.045206jes] All rights reserved.