Bi38Mo7O78 semiconductor nanoparticles were synthesized by sol-gel method. Bi38Mo7O78 keeps the structural characteristics of the patrimonial δ-Bi2O3 structure. Bi38Mo7O78 show an efficient optical absorption in visible light. Photocatalytic property was markedly enhanced for Bi38Mo7O78 nanoparticles. The mechanism of this photocatalysis system was proposed.