In this work, the permittivity measurements of lead magnoniobate PMN (111) single crystals with thicknesses of 400 µm, 25 µm and 10 µm have been carried out at a frequency of 0.1 Hz. The samples in the form of plane-parallel plates were prepared by hand grinding and then their sides were coated with 80 nm Cr-Au electrodes. The topography of the sample surfaces was controlled by AFM. The temperature dependences of the complex permittivity were obtained in the range from 150 to 350 K. The changes in the temperature dependences of ε with varying the thickness of the single crystals were analyzed. The peak on the temperature curve of ε' was shown to shift to a higher temperature with a decrease in the sample’s thickness, and its width increasing significantly. A phenomenological expression was proposed to describe the permittivity ‒ thickness relationship of a single crystal.