Halogen or organophosphorus-based flame retardants have been used as conventionalflame retardants for textile fibers. While halogen-based flame retardants emittoxic gases such as HBr and HCl, some commercial phosphorus-based flame retardantsmay release large amounts of formaldehyde. In this study, as a new ecofriendly organophosphorusflame retardant, vinyl bisphosphonic acid (VBPA) is synthesized from 1-Hydroxyethane-1,1-diphosphonic acid (HEDP) with a yield of 90.8% by a high-temperaturedehydration reaction at 200 °C for 1.5 h, which was confirmed by 31P nuclear magnetic resonance(NMR) as well as Fourier transform infrared (FT-IR) and Raman analyses. In addition,the thermal behavior of the VBPA-treated cotton fabrics are investigated using limitingoxygen index (LOI), thermogravimetric analysis (TGA), and microcombustion calorimetry(MCC). Lowered thermal decomposition and combustion reactions may be resultedfrom the dehydration and crosslinking activated by the phosphonic acids in the VBPAcuredcotton, indicating a condensed-phase flame-retarding mechanism. VBPA can beused as a new formaldehyde-free flame retardant for cellulosic fabrics.