Hollow nanoparticles with large specific surface area and high atom utilization are promising catalysts for the hydrogen evolution reaction(HER).We describe herein the design and synthesis of a series of ultra-small hollow ternary alloy nanostructures using a simple one-pot strategy.The same technique was demonstrated for hollow PtNiCu nanoparticles,hollow PtCoCu nanoparticles and hollow CuNiCo nanoparticles.During synthesis,the displacement reaction and oxidative etching played important roles in the formation of hollow structures.Moreover,our hollow PtNiCu and PtCoCu nanoparticles were single crystalline,with an average diameter of 5 nm.Impressively,ultra-small hollow PtNiCu nanoparticles,containing only 10％Pt,exhibited greater electrocatalytic HER activity and stability than a commercial Pt/C catalyst.The overpotential of hollow PtNiCu nanoparticles at 10 mA cm-2 was 28 mV versus reversible hydrogen electrode(RHE).The mass activity was 4.54 AmgPt-1 at-70 mV versus RHE,which is 5.62-fold greater than that of a commercial Pt/C system(0.81 A mgPt-1).Through analyses of bonding and antibonding orbital filling,density functional theory calculations demonstrated that the bonding strength of different metals to the hydrogen intermediate(H*)was in the order of Pt＞Co＞Ni＞Cu.The excellent HER performance of our hollow PtNiCu nanoparticles derives from moderately synergistic interactions between the three metals and H*.This work demonstrates a new strategy for the design of low-cost and high-activity HER catalysts.