Novel concave Pd nanocrystals with uniform diameter were successfully prepared in the presence of formaldehyde. While the outer surfaces of the as-prepared concave Pd nanocrystals are {111}, the faces concave toward the polyhedral. center are high-surface-energy {110} faces. The degree of concavity and therefore the percentage of {110} of the nanocrystals are tunable by varying the amount of formaldehyde and the reaction temperature. Owing to the existence of active {110} facets, the electrocatalytic activity of the concave Pd nanocrystals displays dependency on their degree of concavity.