Unreported graft copolymer of N,N'-dimethylacrylamide (DMA) with partially carboxymethylated guar gum (CmgOH) has been synthesized and the reaction conditions have been optimized for affording maximum grafting using a potassium peroxymonosulphate (PMS)/thiourea (TU) redox initiators under nitrogen atmosphere. The study of graft copolymerization has been performed to observe maximum value of grafting parameters except percentage of homopolymer by varying the concentrations of DMA, PMS, and TU. The grafting parameters increase continuously on increasing the concentration of DMA from 8 x 10(-2) to 24 x 10(-2) mol dm(-3), PMS from 5 x 10(-3) to 21 x 10(-3) mol dm(-3), and TU from 1.6 x 10(-3) to 4.8 x 10(-3) mol dm(-3). The optimum temperature and time for grafting of DMA onto CmgOH were found to be 35 degrees C and 120 min, respectively. The water-swelling capacity of graft copolymer is investigated. Flocculation property for both coking and non-coking coals is studied for the treatment of coal mine waste water. The graft copolymer is characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 974-981, 2010