Ce/Eu-doped LiBaB₉O₁₅ (LBB) samples were prepared via conventional high temperature solid state reactions. The XRD patterns, crystal structures, luminescence properties, and decay times were investigated systematically. Ce³⁺ ions exist in LBB:xCe³⁺ that were synthesized in a reducing atmosphere and in an air atmosphere. However, we observed Eu²⁺ ions in LBB:yEu²⁺ in a reducing atmosphere and Eu³⁺ ions in LBB:zEu³⁺ in an air atmosphere. LBB:0.05Ce³⁺,yEu²⁺ phosphors synthesized in a reducing atmosphere only possess Ce³⁺ and Eu²⁺ and exhibit a broad excitation band ranging from 350 to 425 nm. A reduction phenomenon of Eu³⁺ → Eu²⁺ and coexistence of Ce⁴⁺, Ce³⁺, Eu²⁺ and Eu³⁺ were observed when LBB:0.05Ce,wEu phosphors were synthesized in an air atmosphere. There are three processes in LBB:0.05Ce,wEu, i.e., energy transfers from Ce³⁺ to Eu²⁺ and from Eu²⁺ to Eu³⁺, and metal–metal charge transfer (MMCT) between Ce³⁺ and Eu³⁺. Moreover, the MMCT process is dominant in LBB:0.05Ce,wEu due to less efficient energy transfer from Ce³⁺ to Eu²⁺. Moreover, the CIE coordinates of LBB:0.05Ce,wEu vary systematically from light blue (0.313, 0.129) to red (0.589, 0.315) for LBB:0.05Ce³⁺,wEu synthesized in air with the changes in Eu ion concentration. Thus, we can control the color by controlling the synthesis atmospheres.