Quality factor (Q: inverse of tan 8), one of the important properties of the microwave dielectric materials, is discussed based on the crystal structure. Dielectric losses (tan 3) are caused by polars generated by disordering of cations and impurities, and also by instability of crystal structure. There are two cases for disordering of cations: one is based on the order-disorder transition; another is based on the cation distribution on the solid solution series. In this paper, a behavior of quality factor was based on the ordering of cations and stability of the crystal structure in the case of the Ba6-3xR8+2xTi18O54 (R = rare earth) solid solutions, and also by impurity of row materials. (C) 2002 Elsevier Science B.V. All rights reserved.