We report nanoscale mechanical heterogeneity of a metallic glass characterized by dynamic force microscopy. Apparent energy dissipation with a variation of ~12%, originating from nonuniform distribution of local viscoelasticity, was observed. The correlation length of the heterogeneity was measured to be ~2.5 nm, consistent with the dimension of shear transformation zones for plastic flow. This study provides the first experimental evidence on the nanoscale viscoelastic heterogeneity in metallic glasses and may fill the gap between atomic models and macroscopic glass properties.