The samples of complex perovskite Ba(Mg1/3Ta2/3)O-3 ( BMT) were synthesized at various sintering temperatures and the Xray diffraction (XRD) patterns and Raman spectra of the samples were collected. Both the structure refinements using the XRD data and the Raman spectra show that the samples sintered above 1500 degrees C with high pellet relative density (>95%) take almost-perfect B-site-ordered structure; while the samples sintered below 1400 degrees C with low pellet relative density (<70%) take the more disordered structure at B-sites. The vibrational modes of 1:2 ordered BMT [P<(3)over bar>m1(164)] were obtained and illustrated by using first-principle calculations. With the assistance of the calculation results, Raman peaks of BMT were assigned as A(1g)(1) (107 cm(-1)), A(1g)(2) (212 cm(-1)), A(1g)(3) (433 cm(-1)), A(1g)(4) (798 cm(-1)), and E-g(1) (104 cm(-1)), E-g(2) (160 cm(-1)), E-g(3) (264 cm(-1)), E-g(4) (386 cm(-1)), E-g(5) (576 cm(-1)). The highly ordered BMT sample at B-site shows longer phonon lifetime and weaker coupling among phonons than the disordered BMT, which probably are the intrinsic reasons for the highly ordered BMT sample to have the low dielectric loss.