Guided-mode resonance (GMR) filters were implemented by combining a monolayer grating made of polyethylene terephthalate (PET) and a slab waveguide (dielectric film) made of quartz, PET, and Teflon. The terahertz (THz) GMRs were measured and simulated for films with thickness change from 0 to 500 µm. As the thickness increased, the transverse-electric (TE) mode such as TE0,1, TE1,1, TE2,1, and TE3,1 appeared sequentially. Because the magnitude and frequency of the GMRs depend on the films' absorption and thickness, we applied GMRs in order to sense the film's thickness of the film. When the GMR filter was measured, resonance depth and frequency changes according to the film thickness were much more sensitive than they were when only the film was measured using THz time-domain spectroscopy (THz-TDS).