Birnessite type manganese dioxides doped with cobalt were synthesized by calcination of a mixture of KMnO₄ and Co(NO₃)₂·6H₂O at 600°C in air. The artificial birnessite was consisted of differently stacked slab phases depending on the cobalt content. These birnessites doped with Co exhibited different ratio of two phases; two-layer hexagonal (2H) and three-layer rhombohedral (3R) whose stacking sequences are different but belong to birnessite. The ratio of 3R/2H phases increased with increasing the Co quantity. Their electrochemical performances in Li cells were investigated in a LiClO₄-propylene carbonate solution. As a result, the Co-doped birnessite demonstrated the smaller charge transfer resistance and higher reversible capacities than Co-free birnessite.