Solid solution system of the double perovskite La2/3TiO3 and the GdFeO3-type perovskite CaTiO3, (1-x)CaTiO3 - xLa(2/3)TiO(3), has been prepared to investigate the crystal structure and the dielectric properties. In this system, the structure changed from pseudo-cubic perovskite to tetragonal double perovskite at x approximate to 0.7, and to orthorhombic double perovskite at x approximate to 0.9. Microwave dielectric measurement showed a decrease in dielectric constant and temperature coefficient of the resonant frequency and an increase in Q value with increasing x. The sample with x=0.96 showed excellent microwave characteristics, especially high Q(2700) along with high epsilon(I)(90) at 10 GHz. In the low frequency dielectric measurement, dielectric relaxation could be observed in the composition range x less than or equal to 0.3. The dielectric relaxation showed strikingly different behaviors depending on the annealing process in oxygen after sintering.