The effect of the grain size on unit‐cell dimensions and dielectric properties has been studied in ceramic lanthana‐substituted lead titanate of the general composition Pb1−αxLaxTiO3+x (1.5−α) with α = 1.20 and x = 0.07 (PLT7) and x = 0.18 (PLT18). The grain size ranges from 0.6 to 11 μm and the effects are most pronounced on the smaller sizes. The tetragonal distortion, characterized by the ratio c/a of the unit‐cell dimensions c and a, decreases by decreasing grain size. This effect becomes relatively smaller by decreasing temperature. A decrease of the grain size leads to a decrease of the maximum value of the dielectric constant ϵ′ at the Curie temperature T′c and to an increase of ϵ′ at lower temperatures. The Curie temperature shifts to a lower temperature while the phase transition is less sharp with decreasing grain size. The results can be discussed in terms of an average value of internal compression stresses in the direction of the c‐axis of the crystal.