In this article, N-Methylol dimethylphos-phonopropionamide ( FR) in combination with a melamine resin (CL), phosphoric acid (PA) catalyst and zinc oxide (ZnO) or nano-ZnO co-catalyst were used (FR-CL-PA-ZnO or nano-ZnO system) to impart flame-retardant property on cotton fabrics. FR-CL or FR-CL-PA-treated cotton specimen showed roughened and wrinkled fabric surface morphology, which was caused by the attack of the FR with slightly acidity. In addition, FTIR analysis showed some new characteristic peaks, carbonyl, CH(2) rocking and CH(3) asymmetric and CH(2) symmetric stretching bands, in the chemical structure of treated cotton specimens. Apart from these, the flame ignited on the flame-retardant-treated fabrics (without subjected to any post-wet treatment) extinguished right after the removal of ignition source. However, FR-CL treated specimens were no longer flame-resistant when the specimens subjected to neutralization and/or home laundering, while FR-CL-PA treated specimens showed opposite results. By using 0.2% and 0.4% of ZnO or nano-ZnO as co-catalyst, the flame spread rate of neutralized and/or laundered test specimens decreased, even the specimens were undergone 10 home laundering cycles. Moreover, flame-retardant-treated cotton specimens had low breaking load and tearing strength resulting from side effects of the crosslinking agent used, while addition of ZnO or nano-ZnO co-catalyst could compensates for the reduction. Furthermore, the free formaldehyde content was dropped when ZnO and nano-ZnO co-catalyst was added in the treatment. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 612-621, 2011