In this study, 100% cotton fabrics' flame retardant properties were improved using commercial nanoclay (Cloisite 20A), clay-based montmorillonite. Flame retardancy and thermal decomposition behavior of the samples were characterized by the vertical burning test, limiting oxygen index (LOI), and thermogravimetric analysis (TGA). The surface morphology of untreated and treated fabric was examined using a scanning electron microscope (SEM). Fourier-transform infrared spectroscopy (FTIR) analysis was carried out for Cloisite 20A, untreated, and coated fabrics. The test results showed that the coating treatment, even at low nanoclay concentration, is enough to have a considerable flame retardant effect. This result was attributed to the barrier effect of the nanoclay.