We report the growth of electrically-resistive vanadium-doped β -Ga 2 O 3 single crystals via the optical floating zone technique. By carefully controlling the growth parameters V-doped crystals with very high electrical resistivity compared to the usual ntype V-doped β -Ga 2 O 3 (n e ≈ 10 18 /cm −3 ) can be synthesized. The optical properties of such high resistive V-doped β -Ga 2 O 3 are significantly different compared to the undoped and n-doped crystals. We study the polarization-dependent Raman spectra, polarization-dependent transmission, temperature-dependent photoluminescence in the optical wavelength range and the THz transmission properties in the 0.2 -2.6 THz range. The V-doped insulating Ga 2 O 3 crystals show strong birefringence with refractive index contrast ∆n of 0.3± 0.02 at 1 THz, suggesting it to be an ideal material for optical applications in the THz region.