Effects of the radius difference (delta R) between A1 and A2 site cation on the crystal structure and the dielectric properties were investigated for Sr5RTi3Nb7O30 (R=La, Nd, Sm, and Eu) tungsten bronze. The tetragonal tungsten bronze structure was determined for all compounds, and the octahedral distortion significantly varied with the composition. In the Ti/Nb(2)O-6 octahedra, besides the off center displacement of the Ti/Nb(2) cations along the c-axis, the four oxygen ions in the equatorial plane were gradually driven to different planes with decreasing R cation radius, while displacement of the Ti/Nb(2) cations in the ab plane decreased. For the Ti/Nb(1)O-6 octahedra, the equatorial plane deviated from the center of the octahedron when the R cation radius became smaller. The ferroelectric transition in Sr5RTi3Nb7O30 was also dominated by the radius difference between A1 and A2 site cation. When the R cation radius became smaller, the dielectric anomaly of Sr5RTi3Nb7O30 changed from relaxor-like behavior into first-order ferroelectric transition. Structural analysis indicated that compounds with larger radius difference between A1 and A2 site cation exhibited severer octahedra distortion, which was helpful for the ferroelectric transition. The low temperature relaxation observed in Sr5RTi3Nb7O30 (R=Nd, Sm, and Eu) was attributed to the freeze-out of polarizability in the ab plane, which originated from the Ti/Nb(2) cation displacement in the ab plane of the Ti/Nb(2)O-6 octahedra.