The development of high-efficiency and non-noble metal hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. In this work, monoclinic tungsten diphosphide (alpha-WP2) and orthorhombic tungsten diphosphide (beta-WP2) particles were synthesized through a phase-controlled solid-state phosphating reaction route via vacuum encapsulation technique, and were evaluated as HER electrocatalysts. Structural characterizations indicate single phase highly crystalline alpha- and beta-WP2 particles with sub-micron sizes were successfully prepared. Both catalysts deliver remarkable catalytic activity for HER with good stability in acidic media. However, alpha-WP2 particles exhibit better catalytic activity than beta-WP2 with a lower overpotential for achieving cathodic current density of 10 mA cm(-2) and a smaller Tafel slope. Combining experimental measurements and theoretical calculations based on density functional theory (DFT), we conclude that the higher catalytic activity of alpha-WP2 over beta-WP2 can be attributed to the less transfer of electron density from W to P due to a longer W-P bond length, the higher electron conductivity and better charge transfer, the lower kinetic energy barrier of H atom adsorption on catalysts surface for HER and longer H-P bond length for effective desorption of H atom to form H-H bond. Our findings may help the development of transition metal poly-phosphide with precise phase controlling technique for applications in hydrogen production. (C) 2016 Elsevier Ltd. All rights reserved.