To improve the efficiency of overall water splitting, highly active and stable bifunctional electrocatalysts are highly desirable. Herein, we fabricated mixed Ni-Co phosphides (Ni1-xCox-P) as bifunctional catalysts for overall water splitting. Structural characterizations indicated that the Ni1-xCox-P catalysts (0 > x > 1) exhibited hierarchical yolk-shelled morphologies, with a total diameter of 1-2 μm. Interestingly, the shell was assembled by numerous nanosheets with a thickness less than 10 nm. The electrochemical measurements indicated that the Ni1-xCox-P catalysts (0 < x < 1) showed significantly enhanced OER and HER activities in comparison to the pure Ni-P and Co-P catalysts, and the highest OER and HER activities were achieved as x = 0.31. To drive a current density of 10 mA cm-2, the Ni0.69Co0.31-P catalyst required an overpotential of 266 mV for OER and 96 mV for HER, respectively. The alkaline water electrolyzer with the Ni0.69Co0.31-P catalysts as the cathode and anode catalysts required a cell voltage of ca. 1.59 V to achieve a current density of 10 mA cm-2, which was comparable to the integrated performance of commercial Pt/C and IrO2. Furthermore, the electrolyzer assembled by the bufictional electrocatalysts showed a more stable performance than one assembled by commercial Pt/C and IrO2 operated at the simialr current density. The superior activity and long-term stability demonstrate that the hierarchical mixed Ni-Co phosphides have promising potential for application in large-scale water splitting.