Abstract We report a theoretical study of the electronic structure and optical properties of the B phase of niobium pentoxide (B‐Nb 2 O 5 ) and tantalum pentoxide (B‐Ta 2 O 5 ) by means of first‐principle calculations. We have used density functional theory along with the revised Perdew–Burke–Ernzerhof (PBEsol) exchange‐correlation functional and the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional. It has been found that these compounds are indirect wide‐gap semiconductors, the calculated gaps for B‐Nb 2 O 5 (B‐Ta 2 O 5 ) are and eV. We have also calculated the frequency‐dependent and static dielectric tensor, the refraction index and the transmittance. The calculated average static dielectric constants of B‐Nb 2 O 5 (B‐Ta 2 O 5 ) are 33.7 (30.9), in good agreement with the available experimental data.