In this work, the variation of peak to valley (PTV) ratios was estimated and compared with the variation of full energy peak area counts (peak ROIs) versus thicknesses of alluminium and copper absorbed layers using Monte Carlo method with MCNP6 code. The experiment and simulation setup included a gamma spectrometer with NaI(Tl) detector, 137Cs gamma point source and absorbed layers of copper and alluminium. From the simulated spectra for different thicknesses of absorbed layers, PTV ratio and peak ROIs were calculated and the spacial resolutions were estimated and compared with each other. The results showed that the slopes of relative PTV–thickness curve are 5.8 times and 2.2 times more than the corresponding slopes of relative peak ROI - thickness curves for Al plate and Cu plate respectively. Minimum detection limits of material thickness by using PTV method are 0.002 ÷ 0.028 mm for alluminium; 0.005 ÷ 0.147 mm for copper which are more effectively than using peak ROI method (0.05 ÷ 0.14 mm for alluminium; 0.06 ÷ 1.90 mm for copper) in range of the material thickness from 0 to 3.5 mm.