Vaterite-type GdBO3:Eu3+ phosphor was first prepared by using a mild hydrothermal method. GdBO3:Eu3+ phosphor was fairly controlled as high purity, homogeneous, stoichiometric, uniform plate morphology and nonaggregation powders. The typical powders were plate shaped and 1.6-2.0mm in size. As the synthesis temperature was below 300 C, the recovered phosphor had never been subjected to the phase transition to the calcite type during the chemical reaction. The phosphor showed the excitation band peaked at about 239nm, and the emission band peaked at about 591nm had twice the intensity compared with the conventional phosphor. The excitation and emission peaks were somewhat shifted to shorter wavelength. The GdBO3:Eu3+ phosphor showed maximum intensity in emission for the doped 20 at.% of Eu3+ upon 239nm excitation and 10at.% Eu3+ under 146nm excitation, respectively. Therefore, the energy relaxation pathway to the Eu3+ sites should be different. Also, the partial substitution of Y3+ for Gd3+ was very convenient to improve the emission intensity of Eu3+ under 146nm excitation, whereas the emission intensity was decreased with increasing Y3+ concentration under 239nm excitation.