A structural phase transformation from hexagonal to monoclinic has been found in an ordered perovskite compound (Ba, Sr)(Mg1/3Ta2/3)O3, (BSMT). The transformation temperature of BSMT increases and the dielectric constant also changes from 24 to 27 with an increase in Sr concentration. The temperature coefficient of the dielectric constant is also changed with the variation of Sr content. In order to apply BSMT for practical use of dielectric resonators these changes in the dielectric properties, which is dominated by the phase transformation, must be controlled. In order to clarify the relation between the dielectric constant change and the structural phase transformation, phonon mode of BSMT has been studied using Raman spectroscopy. Change in the Raman spectrum of BSMT is classified into four steps. The first and the fourth steps correspond to the phase transformation. The change in the phonon mode detected by Raman spectroscopy is discussed in connection with the structural change.