Photoluminescence of the structures with multiple GaN/AlN quantum wells grown by molecular beam epitaxy has been investigated. The calculated dependence of the energy of the quantum well photoluminescence band peak on the GaN layer thickness has been compared with the experimental data for various ratios of the thickness of the GaN and AlN layers. The thicknesses of the GaN and AlN layers were determined by transmission electron microscopy. The calculations of the photoluminescence band peak energy were carried out in the 6-band kp -approximation with consideration of spontaneous and piezoelectric polarizations. According to the calculation results, the slope of the dependence of the emission energy on the GaN layer thickness decreases as the ratio of the GaN layer thickness to the AlN layer thickness increases in accordance with the decrease in the electric field in the GaN layer. In quantum wells of sufficiently large thickness, the observed photoluminescence band peak energy is higher than the calculation result for undoped structures due to unintentional doping of the quantum wells, which leads to screening of the built-in electric field.