The band structure and effective mass of disordered chalcopyrite photovoltaic materials Cu_1−xAg_xGaX₂ (X = S, Se) are investigated by density functional theory. Special quasirandom structures are used to mimic local atomic disorders at Cu/Ag sites. A local density plus correction method is adopted to obtain correct semiconductor band gaps for all compounds. The bandgap anomaly can be seen for both sulfides and selenides, where the gap values of Ag compounds are larger than those of Cu compounds. Band gaps can be modulated from 1.63 to 1.78 eV for Cu_1−xAg_xGaSe₂, and from 2.33 to 2.64 eV for Cu_1−xAg_xGaS₂. The band gap minima and maxima occur at around x= 0.5 and x= 1, respectively, for both sulfides and selenides. In order to show the transport properties of Cu_1−xAg_xGaX₂, the effective mass is shown as a function of disordered Ag concentration. Finally, detailed band structures are shown to clarify the phonon momentum needed by the fundamental indirect-gap transitions. These results should be helpful in designing high-efficiency photovoltaic devices, with both better absorption and high mobility, by Ag-doping in CuGaX₂.