Nanostructured L1 0 Co-Pt thin films with high perpendicular coercivity have been successfully synthesized on the substrate of Pt buffer layer/Cr seed layer/Si(0 0 1) by an electrodeposition process from a novel plating bath containing ammonium tartrate and ammonia solution as complexing agent and by subsequent annealing. Results show that the alloy compositions can be readily controlled by adjusting the metal ions ratio in the plating bath and applied current density. The as-deposited films with nanocrystalline structure contain a disordered face-central cubic (fcc) structure. By annealing above 600℃, the films with the disordered phase structure transform to L1 0 ordered phase structure. The nanostructured L1 0 ordered Co-Pt thin films exhibit high perpendicular coercivity comparable to traditional magnetic recording media. The present synthesis provides a unique and effective electrochemical means for inexpensively preparing L1 0 ordered Co-Pt thin films as new magnetic recording media from the aqueous solution.