SrSmAlO4 microwave dielectric ceramics were modified by Sr/Ti cosubstitution for Sm/Al. The effects of radius difference of A‐site ions on the microwave dielectric characteristics were investigated together with the structure. Sr1+xSm1−xAl1−xTixO4 (x=0, 0.05, 0.10, and 0.15) ceramics were prepared by a solid‐state reaction approach. X‐ray diffraction studies revealed a single‐phase K2NiF4‐type solid solution with corresponding peaks shifting to lower 2θ as x increased. Minor inhomogeneous grain morphology for x=0.05 and a trace amount of second phases for x=0.10, 0.15 were detected by backscattered‐electron microscopy and energy‐dispersive X‐ray analysis. With increasing Sr/Ti cosubstitution, the dielectric constant ɛr increased from 18.4 to 20.4, and the temperature coefficient of resonant frequency τf was adjusted from −1.8 to 7.4 ppm/°C almost linearly. However, the Q×f value decreased from 74,500 GHz at x=0–53,000 GHz at x=0.15. The internal stresses caused by the decreased tolerance factor and the large ionic radii difference between Sr2+ and Sm3+ should be the predominant reasons for such a decrease in the Q×f value. The high‐resolution transmission electron microscopic results revealed an increase in the lattice distortion with increasing Sr/Ti cosubstitution, and subsequently supported the above conclusion upon the increased internal stresses.