The structure and dielectric properties of Sm1.5Sr0.5Ni1−xAlxO4 (x = 0.3, 0.5) ceramics were presented. A single orthorhombic phase was obtained in Sm1.5Sr0.5Ni0.7Al0.3O4 ceramics, while minor impurities existed in Sm1.5Sr0.5Ni0.5Al0.5O4 ceramics. Temperature stable giant dielectric response was observed in these two ceramics, and only one dielectric relaxation was found. After comparing the activation energies of dielectric relaxation and electrical conduction, the giant dielectric response should be attributed to the adiabatic small polaronic hopping process, and the dielectric behaviour of the samples with different aluminium contents confirmed the correlation between the dielectric response and electrical conduction.