polymethylmethacrylate (PMMA) was synthesized by free radical addition polymerization using high-power ultrasound technique in presence of peroxodisulphate  as initiator, then reacted with different concenterations of phosphoric acid. Montmorillonite (MMT) clay was modified using Cetyl Trimethyl Ammonium Bromide (CTAB).This study aims to investigate the effect of different concentrations of phosphoric acid used in polymer/clay modification on the fire-retardant properties of treated cotton fabrics. Firstly, the prepared polymer/clay nanocomposite and the cotton fabrics (either treated or untreated) were characterized using different physico-chemical techniques viz., FTIR spectra, Scanning Electron Microscope (SEM), SEM-EDX and Transmission Electron Microscope (TEM). The thermal stability of the prepared phosphorous polymer/clay nanocomposite was investigated using TGA and DSC analysis. Flammability tests and the fire-retardant properties of treated and untreated cotton fabrics were evaluated by determining the rate of burning and Limited Oxygen Index (LOI) % values. The tensile strength of treated and untreated cotton fabrics were determined The data revealed that, increasing concentration of phosphoric acid led to increase the flammability resistance more than the untreated cotton fiber and all treated samples showed lower burning rate than that of the untreated one.