Thanks to the availability of single crystal electrodes and the advance of scanning probes, it is now possible to characterize the process of electrodeposition in real time and the atomic structure of ultrathin film. Reported here is an in situ scanning tunneling microscopy (STM) study of cobalt electrodeposition on an ordered Pt(1 1 1) electrode in a pH 3 chloride-containing medium. It was found to be a two-staged process, involving an underpotential deposition (UPD) stage and an overpotential deposition (OPD) occurring at potentials positive and negative of the Nernst potential of -0.57 V (versus Ag/AgCl) calculated for 0.04 M CoCl2. Time-dependent STM imaging revealed Co nucleated mainly in one atom high islands on terraces and at steps in the UPD stage, followed by quasi layer-by-layer growth to form a smooth thin film up to five layers in thickness. The UPD layer comprised Co atoms arranging in disarray, possibly resulting from competitive adsorption of hydrogen at the Pt electrode. Starting from the second layer, the Co film assumed ordered microstructures featuring a long range undulation of atomic height. This moire structure consisted of Co(0 0 0 1) like plane stacked on the Pt(1 1 1) substrate with an in-plane interatomic spacing of 0.257 nm in the bilayer film. Due to different lattice constants of Co and Pt, the Co/Pt(1 1 1) was strained, but the degree of this interfacial strain gradually decreased with the thickness of Co deposit, as revealed by the diminish of the moire pattern when the Co film grew to more than five layer in thickness. Meanwhile, the morphology of the Co deposit became rougher with thickness, which suggests a Stranski-Krastanov growth.