Halogen or organophosphorus-based flame retardants have been used as conventional flame retardants for textile fibers. While halogen-based flame retardants emit toxic gases such as HBr and HCl, some commercial phosphorus-based flame retardants may release large amounts of formaldehyde. In this study, as a new ecofriendly organophosphorus flame retardant, vinyl bisphosphonic acid (VBPA) is synthesized from 1- Hydroxyethane-1,1-diphosphonic acid (HEDP) with a yield of 90.8% by a high-temperature dehydration 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 limiting oxygen index (LOI), thermogravimetric analysis (TGA), and microcombustion calorimetry (MCC). Lowered thermal decomposition and combustion reactions may be resulted from the dehydration and crosslinking activated by the phosphonic acids in the VBPA-cured cotton, indicating a condensed-phase flame-retarding mechanism. VBPA can be used as a new formaldehyde-free flame retardant for cellulosic fabrics.