The effect of Ba( $$\hbox {Mg}_{1/8}\hbox {Nb}_{3/4})\hbox {O}_{3}$$ Mg 1 / 8 Nb 3 / 4 ) O 3 phase on structure and dielectric properties of $$\hbox {Ba(Mg}_{1/3}\hbox {Nb}_{2/3})\hbox {O}_{3}$$ Ba(Mg 1 / 3 Nb 2 / 3 ) O 3 was studied by synthesizing $$(1{-}x)\hbox {Ba(Mg}_{1/3}\hbox {Nb}_{2/3})\hbox {O}_{3}{-}x\hbox {Ba}(\hbox {Mg}_{1/8}\hbox {Nb}_{3/4})\hbox {O}_{3}$$ ( 1 - x ) Ba(Mg 1 / 3 Nb 2 / 3 ) O 3 - x Ba ( Mg 1 / 8 Nb 3 / 4 ) O 3 ( $$x = 0$$ x = 0 , 0.005, 0.01 and 0.02) ceramics. Superlattice reflections due to 1:2 ordering appear as low as $$1000^{\circ }\hbox {C}$$ 1000 ∘ C . $$\hbox {Ba}(\hbox {Mg}_{1/3}\hbox {Nb}_{2/3})\hbox {O}_{3}$$ Ba ( Mg 1 / 3 Nb 2 / 3 ) O 3 forms solid solution with $$\hbox {Ba}(\hbox {Mg}_{1/8}\hbox {Nb}_{3/4})\hbox {O}_{3}$$ Ba ( Mg 1 / 8 Nb 3 / 4 ) O 3 for all ‘x’ values studied until $$1350^{\circ }\hbox {C}$$ 1350 ∘ C . Ordering was confirmed by powder X-ray diffraction pattern, Raman study and HRTEM. Ceramic pucks can be sintered to density $${>}92\%$$ > 92 % of theoretical density. Temperature and frequency-stable dielectric constant and nearly zero dielectric loss (tan $$\delta $$ δ ) were observed at low frequencies (20 MHz). The sintered samples exhibit dielectric constant ( $$\varepsilon _{\mathrm{r}})$$ ε r ) between 30 and 32, high quality factor between 37000 and 74000 GHz and temperature coefficient of resonant frequency ( $$\tau _{\mathrm{f}})$$ τ f ) between 21 and $$24\hbox { ppm }^{\circ }\hbox {C}^{-1}$$ 24 ppm ∘ C - 1 .