Nanocrystalline PbBi 2 Nb 2 O 9 was successfully prepared by a modified sol–gel method. The as-prepared nanocrystalline PbBi 2 Nb 2 O 9 was an active visible-light-driven photocatalyst for the photoreduction of 4-nitroaniline to p-phenylenediamine in aqueous solution upon purging with N 2 . Its photocatalytic activity was determined by the balance between the surface area and crystallinity. The best sample was obtained after calcination at 650°C, giving 4-nitroaniline conversion of ∼100% and p-phenylenediamine selectivity of ∼99% after irradiation for 150min, which was much higher than those of bulk PbBi 2 Nb 2 O 9 and TiO 2− x N x photocatalysts. Further experiments revealed that oxalate anion (as a hole scavenger) was indispensable for the photocatalytic reduction of 4-nitroaniline. Moreover, on the basis of the electron spin resonance analysis result, a mechanism was proposed to explain the photoreduction of 4-nitroaniline over nanocrystalline PbBi 2 Nb 2 O 9 .