The microwave dielectric properties and microstructure of Mg(3)B(2)O(6) ceramics with the addition of 0-5 wt% Mg(2)B(2)O(5) were investigated. Scanning electron microscopy of the Mg(3)B(2)O(6) ceramics sintered at 1310 degrees C revealed the presence of anisotropic, exaggeratedly grown grains with a crystallographic fault that extends along the direction of the anisotropy. The most pronounced, exaggerated grain growth was observed for Mg(3)B(2)O(6) with 5 wt% of Mg(2)B(2)O(5). The anisotropic grains were examined by electron backscatter diffraction. The analysis of the Kikuchi pattern showed that the crystallographic fault is a (011) twinning plane. The duration of annealing was found to have no significant effect on the dielectric permittivity, while the Q x f values increased two times with a prolongation of the annealing from 20 to 160 h. The highest Q x f value of 240 000 GHz was measured for Mg(3)B(2)O(6) ceramics with 5 wt% of Mg(2)B(2)O(5). These ceramics consisted of 10 times larger grains than the Mg(3)B(2)O(6) ceramics without the added Mg(2)B(2)O(5), which also showed high Q x f values of 200 000 GHz. The decrease of the amorphous phase during the annealing could be one of the reasons for this improvement of the Q x f values.