First principles calculations are performed to comparatively reveal the fundamental effects of Ag alloying on phase stability, hydrogen solubility, hydrogen diffusivity, and hydrogen permeability of body-centered-cubic (BCC) and face-centered-cubic (FCC) Pd8Cu8 phases. It is found that the addition of Ag would slightly decrease phase stability of Pd8Cu8, and the BCC structure of Pd8Cu7Ag and Pd7Cu8Ag is energetically more stable than its FCC counterpart at 0 K. Calculations also reveal that crystal structure (BCC or FCC) of PdCuAg and substituting position (Pd or Cu) of Ag should play dominant roles in determining the relative magnitude of hydrogen solubility, hydrogen diffusivity, and hydrogen permeability between PdCuAg and PdCu phases. The present results agree well with experimental observations in the literature, and provide a deep understanding of the fundamental effects of Ag on hydrogen behaviors in PdCu phases.