Uniform and monodisperse Gd(OH)₃:Eu³⁺ hexagonal prisms were successfully synthesized at mild conditions via a large-scale and facile homogeneous coprecipitation process without using any catalysts, surfactants or templates. The size of the as-formed Gd(OH)₃:Eu³⁺ precursor prisms could be modulated from the micro- to nanoscale by the use of urea and changing the pH values of the initial solutions. A possible formation mechanism for the Gd(OH)₃:Eu³⁺ hexagonal nano-/microprisms was proposed. After a postannealing process, Gd₂O₃:Eu³⁺ hexagonal nano-/microprism phosphors with a slight shrinking in size can be transformed from Gd(OH)₃:Eu³⁺. Both the Gd₂O₃:Eu³⁺ nanoprisms and microprisms exhibit the same strong red emission corresponding to the ⁵D₀ → ⁷F₂ transition (610 nm) of Eu³⁺ under UV light excitation (243 nm) and low-voltage electron beam excitation (1–6 kV). Furthermore, the experimental results indicate that the luminescence properties of the as-obtained phosphors are dependent on their morphologies and sizes. As a result of the controllable morphology and size, and excellent luminescence properties, these Gd₂O₃:Eu³⁺ nano-/microprism phosphors may find potential applications in optoelectronic devices (fluorescent lamps and field emission displays), bioanalysis and biomedical areas and so on.