Ultrafine crystalline materials have been extensively investigated as high-rate lithium-storage materials due to their shortened charge-transport length and high surface area. The pseudocapacitive effect plays a considerable role in electrochemical lithium storage when the electrochemically active materials approach nanoscale dimensions, but this has received limited attention. Herein, a series of (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O electrodes with different particle sizes were prepared and tested. The ultrafine (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O nanofilm (3-5 nm) anodes show a remarkable rate capability, delivering high specific charge and discharge capacities of 829, 698, 602, 498 and 408 mA h g -1 at 100, 200, 500, 1000 and 2000 mA g -1 , respectively, and a dominant pseudocapacitive contribution as high as 90.2% toward lithium storage was revealed by electrochemical analysis at a high scanning rate of 1.0 mV s -1 . This work offers an approach to tune the lithium-storage properties of (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O by size control and gives insights into the enhancement of pseudocapacitance-assisted lithium-storage capacity.