Structures and microwave dielectric properties of Ba2-2xSr2xSmSbO6 (x = 01) ceramics were studied in this article. The phase transitions were analyzed using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopy. All structures are proved to be ordered double perovskites, and a series of composition-induced phase transitions are confirmed in the Ba2-2xSr2xSmSbO6 system via the sequence: Fm-3m ? I2/m ? P21/n. Dielectric properties of the ceramics were measured at microwave frequencies (911 GHz). With the increase of x, the dielectric constant (er) varies gradually from 20.7 to 16.0, the quality factor (Q x f) decreases first from 86 500 GHz to a minimum value at x = 0.4 and then increases slightly to 93 000 GHz, and the temperature coefficient of resonant frequency (tf) decreases from +25 to -50 ppm/degrees C. The relationships between composition, structure, and microwave dielectric properties in Ba2-2xSr2xSmSbO6 were discussed from the phase transition consideration.