Prussian blue compounds have potential advantages of high working potentials and multi-electron redox capacities as Na-ion battery cathodes; however, they suffer from their poor capacity utilization in aqueous electrolytes due to the blockage of the active lattice sites by coordinating and zeolitic water for Na insertion reaction. To overcome this problem, we synthesized low-defect Prussian blue FeFe(CN)6 nanocrystals and revealed their Na-storage performance in aqueous electrolytes. Benefitting from its suppressed lattice defects and well-defined nanocubic morphology, the as-prepared Prussian blue nanocrystals exhibit a greatly improved capacity of ~125mAhg−1, a strong rate capability with 102mAhg−1 at a high rate of 20C and a remarkably long cycle life with 83% capacity retention over 500 cycles, exceeding considerably the aqueous Na-storage cathodes reported so far and capably serving as a high performance cathode for aqueous Na-ion batteries. As a model compound, the superior electrochemical performance arising from the low-defect Prussian blue nanocrystals provide a new insight into the design of high capacity Na-storage host materials for large scale electric storage applications.