Manganese-based layered KxMnO2 (x = 0.3 and 0.45) cathodes with P2/P3-type structure for K-ion batteries are prepared through a simple co-precipitation method. It is discovered that the amount of K not only plays an important role in the structure and morphology of the samples, but also has a certain effect on the electrochemical performance. In spite of the same synthesis conditions, the K0.45MnO2 sample displays different structure and smaller particle sizes in comparison with that of the K0.3MnO2 sample. The redox potentials of the two samples are almost the same. Both of them present excellent cycle stability and rate performance when tested as cathodes for K-ion batteries. However, K0.45MnO2 shows slightly better performance than that of K0.3MnO2. It can deliver a reversible specific capacity of 128.6 mAh g(-1) at 20 mA g(-1) with an average voltage of 2.75 V vs. K/K+. In addition, the K0.45MnO2 exhibits a fast rate performance with a specific capacity of 51.2 mAh g(-1) even at 200 mA g(-1), which exceeds the performance of some reported metal-based oxide cathodes. These results may provide new insight into developing manganese-based oxides cathode materials for Kion batteries.