Ferroelectric transition and the low-temperature dielectric relaxations were investigated in Sr4(La1-xSmx)2Ti4Nb6O30 ceramics. The dielectric nature of the present ceramics was governed by the radius difference between A1- and A2-site ions. Low-temperature dielectric relaxations were indicated for all the compositions, and the diffuse ferroelectric peak appeared gradually with increasing x (increasing ion radius difference between A1- and A2-sites), and it became the normal ferroelectric one for x = 1. There was no peak on the differential scanning calorimetry (DSC) curves for the compositions with x = 0, 0.25, 0.5, and 0.75, while an obvious endothermic peak was observed for x = 1. Based on the analysis of the dielectric data and the DSC results, the ferroelectric transition for x = 1 was determined as a first order phase transition, and the second order phase transition was suggested for other compositions.