The development of high-performance electrocatalysts for alkaline hydrogen evolution reaction (HER) is an important step towards achieving hydrogen economy but remains major challenge. This study reports the growth of ultralow Ru (1 wt%) modified Co nanostructures (Ru-Co) with coral-like nanoarray structures on carbon cloth via lattice expansion tactics. Specifically, the as-fabricated Ru-Co electrode exhibits extraordinary electrocatalytic activity, with an overpotential of 25 mV at a current density of 10 mA cm and a small Tafel slope of 31 mV dec in alkaline media, which is superior to the benchmark Pt/C. Density functional theory (DFT) calculations disclose that Ru can shift the d-band center of Co to higher fermi level, which promotes electron transfer, HO dissociation and *H adsorption during catalysis. This work provides a promising strategy for the development of efficient transition metal-based alkaline HER electrocatalysts via accelerating electron transfer and promoting mass transportation.