Bi1-xSmxFe1-yMnyO3 (BSFMO, x = 0.0, 0.05; y = 0.0, 0.05, 0.10, 0.15, 0.20, 0.25) nanoparticles were synthesized by using double solvent sol-gel method. Photocatalytic activity was investigated under UV and visible-light illumination. The structural, morphological, and optical properties were analyzed by X-ray diffraction, scanning electron microscopy, and UV-vis spectroscopy respectively. The crystallite size of BiFeO3 (BFO) decreases from (57.3-17.2 nm) with the increase in Sm and Mn-doping concentration. The surface morphology shows that the pure and Sm-doped BFO nanoparticles are irregular in shape but changes to spherical shape after Mn-doping up to 25%. The band-gap engineering of BFO nanoparticles is achieved by co-doping of Sm and Mn. The band-gap of BFO could be tuned successfully from 2.08-1.45 eV, which may be due to the distortion induced in Fe-O octahedron and the rearrangement of molecular orbitals. These results give rise to enhanced photocatalytic activity by degradation of organic dyes (MB, CR, and MV) under the visible-light illumination.