The structural evolution of Sr1−xSm1+xAl1−xTixO4 (x=0, 0.05, 0.10, and 0.15) microwave dielectric ceramics was investigated together with its effects on dielectric loss. Through the analysis of Rietveld refinement of powder X-ray diffraction, the normalized bond length and normalized bond valence showed an abnormal variation of Sr/Sm–O(2b) and Al/Ti–O(2) bonds from x=0.10 to x=0.15. The abnormal variation related to these bonds in Raman spectra and IR reflectivity spectra was observed. IR reflectivity spectra were calculated by the classical oscillator model fitting to evaluate the intrinsic loss. The calculated Q × f value and the measured ones both decreased as x increased, and a sharp decrease from x=0.10 to x=0.15 was observed. The decreased tolerance factor should be responsible for the decreased Q × f value. Moreover, the abnormal evolution of Sr/Sm–O(2)–Al/Ti bonding that can cause an extra local stress should be responsible for the sharp decrease from x=0.10 to x=0.15 in Q × f value.