YBO3:Eu3+ nanoparticles have been prepared by hydrothermal method. The size and morphology of the nanoparticles were controlled by the reaction conditions. The luminescence of the nanoparticles has been investigated and compared to that of the bulk YBO3:Eu3+. In nano-sized materials, a large portion of doped rare earth ions located at the sites with micro-environments are affected by the surface. Such an influence could enhance the Judd-Ofelt Ω2 parameter, and hence increase the relative intensity of the Eu3+5D0→7F2 emission and improve the chromaticity of the red phosphor.