PULSED-co-electrodeposition (PCD) is a new and promising technique for producing alloymaterials of nanometer grain size. In this letter, Co-Ni alloys were produced by PCD, and theeffect of CO~(+2) ion concentration in the depositing baths on the Co content and the microstruc-ture in the corresponding deposit was studied by the position sensitive atom probe (PoSAP),TEM and X-ray diffraction. The Co-Ni phase diagram shows that, at room temperature,there are two phases in Co-Ni alloys, one is εCo with hcp lattice, the other is αCo of fcc lat-tice. A Co-Ni alloy with a Ni content below 27% consists of single εCo, beyond 36% consistsof single αCo, in between consists of εCo + αCo. The examination results of TEM, SEM andX-ray diffraction indicated that the Co-Ni deposits with average grain size of 70 nm and thick- PULSED-co-electrodeposition (PCD) is a new and promising technique for producing alloy materials of nanometer grain size. In this letter, Co-Ni alloys were produced by PCD, and the effect of CO ~ (+2) ion concentration in the depositing baths on the Co content and the microstruc-ture in the corresponding deposit was studied by the position sensitive atom probe (PoSAP), TEM and X-ray diffraction. The Co-Ni phase diagram shows that at room temperature, there are two phases in Co-Ni alloys, one is εCo with hcp lattice, the other is αCo of fcc lat-tice. A Co-Ni alloy with a Ni content below 27% consists of single εCo, beyond 36% consists of single αCo, in between consists of εCo + αCo. The examination results of TEM, SEM and X-ray diffraction indicated that the Co-Ni deposits with average grain size of 70 nm and thick-