In this study, flame retardant properties of 100% cotton fabrics were tried to be improved by using commercial nanoclay (Cloisite 20A) which is a 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 results of the tests 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.