The structural evolution and microwave dielectric properties of (1 À x)Ca 0:8 Sr 0:2 TiO 3 -xLi 0:5 Sm 0:5 TiO 3 (x ¼ 0 {1:0) solid solutions were investigated. Most samples exhibit the single orthorhombic perovskite structure and a homogeneous microstructure with high density except for the composition with x ¼ 0:5 which included a small amount of the TiO 2 phase. Increasing Li 0:5 Sm 0:5 TiO 3 content results in a structural change in symmetry around x ¼ 0:8. A-site ordering structures were observed in compositions with x ! 0:80. The dielectric constant (" r ) decreased with increasing x. The quality factor (Q Â f ) decreased first up to x ¼ 0:5 with increasing x, then increased again up to x ¼ 0:82, and then decreased as x increased to x ¼ 1:0. The temperature coefficient of the resonant frequency ( f ) decreased from high positive values to negative values with increasing x. A near zero f was achieved at x ¼ 0:81. Optimized microwave dielectric properties were estimated to be " r ¼ 95, Q Â f ¼ 3303 GHz, and f ¼ À1:8 ppm/ C at x ¼ 0:81.