Ln(IO3)(3) (Ln = Ce, Nd, Eu, Gd, Er, Yb) polycrystals were hydrothermally synthesized using lanthanide nitrate or lanthanide oxide and iodic acid as precursors. X-ray diffraction was used to characterize the crystal structures of the Ln(IO3)(3) products. Scanning electron microscopy was carried out to observe the microscopic morphologies. The lattice spacings were studied by high-resolution transmission electron microscopy and selected area electron diffraction. We evaluated the photocatalytic efficiency by decomposing methyl orange dye under ultraviolet light irradiation, and the Ln(IO3)(3) products show excellent photocatalytic properties. To rule out the effect of photosensitization, 2,4-dichlorophenol was also photodegraded. As one of the key factors of photocatalysis, ultraviolet visible diffuse reflectance spectra of the Ln(IO3)(3) samples were also studied, and all products have strong absorption in the ultraviolet region.