A simple ultrasound-assisted method involving polyvinyl alcohol (PVA) was employed to prepare the size controlled bismuth oxide and bismuth hydroxide nanostructures. The XRD study confirms the structure of the system as cubic with β-Bi2O3 phase. Scanning electron microscopy revealed that the β-Bi2O3 and bismuth hydroxide samples composed of cube like structures. The SEM micrographs also show the improvement in the particle size distribution upon PVA capping agent addition. Absorption spectra have been obtained using a UV-Vis spectrophotometer to find the optical direct band gap. The optical gap of pure and PVA capped bismuth oxide and bismuth hydroxide nanomaterials have been calculated using the Tauc’s relationship and blue shift in the optical gap has been reported. We also found that the optical band gap (Eg) increases with the increase in molar concentration of the PVA. Fourier transform infrared (FT-IR) spectra of the prepared samples show the presence of characteristic bands for the PVA in the spectrum and reveal the capping effect of PVA on bismuth oxide and hydroxide nanostructures.