Nanosize Polyreactive blue MXR dye was synthesized from reactive blue MXR dye in presence of potassium persulfate as catalyst. The formation of polyreactive blue MXR was indicated by colour change from blue to brown. The characterization techniques such as, Fourier transform infrared spectroscopy (FTIR), Atomic force microscopy (AFM), and X-ray diffractrometry (XRD) were used to characterize the formation of nanosize polyreactive blue MXR. The absence of asymmetric stretching of NH2 group in polymer dye FTIR spectrum confirmed the polymerization of dye was occurring. The average particle size of the polymer dye was found to be 18.11 nm according to Scherer formula.AFM analysis shows the three dimensional structure of polyreactive blue MXR.