An unreported graft copolymer of guar gum with N-vinyl formamide has been synthesized using potassium bromate/ascorbic acid redox pair. By studying the effect of the concentration of monomer, bromate ion, ascorbic acid, guar gum along with the effects of time and temperature on the grafting characteristics: grafting ratio (%G), add on (%A), conversion (%C), efficiency (%E), homopolymer (%H) and rate of grafting (Rg), the reaction conditions for optimum grafting have been determined. The maximum grafting ratio, add on, conversion, efficiency and rate of grafting were obtained at minimum concentration of N-vinyl fomamide (NVF) i.e. at 10x10(-2) mol dm(-3). The maximum values of these parameters were observed at minimum temperature (35 degrees C) and minimum reaction time (120 min). On increasing the concentration of ascorbic acid from 0.4x10(-3) to 2.0x10(-3) mol dm(-3) the above parameters show increasing trend while on increasing the concentration of bromate ion from 0.2 to 1.8x10(-2) mol dm(-3) the parameters show decreasing trend. Infrared spectra of Guar gum and guar gum-g-NVF has been investigated. Thermogravimetric analysis and differential scanning calorimetric analysis showed that guar gum-g-NVF was thermally more stable than guar gum.