Red phosphor BaSiF6:Mn4+ has been synthesized by a hydrothermal method at 120 degrees C for 24 h, in which either Si or SiO2 is used as silicon source. HF as weak acid performs a complex agent for the formation of anion groups [SiF6](2-) and [MnF6](2-). The luminescence properties of undoped BaSiF6 have been firstly observed. The dependence of luminescence intensities of BaSiF6:Mn4+ on the concentrations of HF and KMnO4 in precursory solution has been investigated. The as-prepared BaSiF6:Mn4+ exhibits high chemical stability even in deionized water.