A co-precipitation method is proposed for the synthesis of highly crystalline BiFeO3 (BFO) nanoparticles with varying Bi/Fe molar ratio (0.9, 1.0, 1.1) after calcination at 550 ◦C and 600 ◦C in air. The effect of the varying Bicontent on the phase constitution, structural and multiferroic properties of the nanoparticles is investigated. Differential thermal analysis discloses that the crystallization of the initially, amorphous, powder occurs at 484 ◦C, whereas the Curie temperature is 820 ◦C. X-ray diffraction patterns and Fourier transform infrared spectra evince the formation of the rhombohedral structure with R3c space group symmetry in all samples, whereas Rietveld refinement confirms the phase purity of the 600 ◦C calcined BFO at Bi/Fe = 1.0. Transmission electron microscopy indicates that the average particle size is tuned with respect to calcination temperature and Bi content, giving rise to the enhanced magnetic properties of the 550 ◦C calcined BFO 0.9 at room temperature due to size effects and an increment of structural distortions. By rising the Bi-content, the surplus Bi stabilizes the highly polar R3c rhombohedral structure at both calcination temperatures, resulting in the enhanced values of the real (ε′ ) dielectric constant in BFO 1.0 and BFO 1.1 at room temperature. UV–visible absorbance spectra of both 550 ◦C and 600 ◦C calcined BFO 1.0 exhibit strong visible light absorption, while the corresponding Tauc’s plots manifest that the optical band gap energy of BFO nanoparticles is about 2 eV following particle size trends.