Solute H in Ti alloys has an important effect on their deformation and ductility, therefore changing the mechanical properties and improving the mechanical processing. We have performed first-principles calculations to investigate the effects of H on the mechanical properties of the hexagonal close packed alpha-Ti and the body centered cubic beta-Ti, and make an attempt to understand how the solute H improves the mechanical processing. We compute the structural parameters, elastic properties, and the generalized stacking fault energies for alpha- and beta-Ti with and without the H addition. We find H decreases the shear moduli and the unstable stacking fault energies of alpha-Ti, enhancing the deformation tendency, while these quantities are increased by H in beta-Ti. We predict the H effect on the ductility using different criteria, and find H makes alpha-Ti more ductile, but raises the brittleness of beta-Ti. Our results indicate H may have a favorable effect on improving the mechanical processing of Ti alloys. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.