In this study, Mg-doped LiNbO3 nanoparticles are prepared and their photocatalytic ability is studied for bioorganic degradation application. It is found that their photocatalytic ability shows a pronounced dependence on the Mg-doping levels, and in particular an unusual threshold effect exists at the 2-mol%-doping level. Moreover, X-ray diffraction (XRD), SEM, and differential scanning calorimetry (DSC) analyses show that the similar threshold effect also exists in the Mg-doping dependences of the crystalline anisotropy, morphology, and endothermic behavior of these LiNbO3 nanoparticles. The relevance between the basic properties and the photocatalytic activity of the LiNbO3 nanoparticles is discussed, and their Mg-doping dependences are finally connected with the (Nb-Li)(4+) and (Mg-Nb)(3-) defects in the lattice. It is suggested that both the morphology and the lattice defects of LiNbO3 nanoparticles contribute to the photocatalytic activity and that trapping holes is the essential way through which the (Mg-Nb)(3-) defects make the contribution.