Cobalt nickel double hydroxides (Co x Ni1−x LDHs) nanoparticles were synthesized by a polyvinyl pyrrolidone-assisted chemical co-precipitation method. It was found that the ratio between Co and Ni had a significant effect on their electrochemical activities. The specific capacitance of 2614Fg−1 was achieved when the atomic ratio of Co to Ni was equal to 0.57:0.43. This Co0.57Ni0.43 LDH sample had a BET specific surface area of 109.8m2 g−1 and total pore volume of 0.6036cm3 g−1, and its pore size distribution was mainly in the range of 1.5–5 nm, which led to the high liquid–solid interfacial area and further increased the utilization of the active materials. Additionally, the Co0.57Ni0.43 LDH electrode exhibited a good electrochemical stability with 86.4% of the initial capacitance over 1000 consecutive cycles at 5Ag−1.