Effect of solvent viscosity on the longest rotational relaxation time of xanthan molecule has been examined using a single molecule tracking method. Incorporating inverted epi-fluorescence microscope and charged-coupled device (CCD) camera, various features of xanthan (i.e., radius of gyration, orientation angle, etc.) were interpreted by image processing algorithm from the captured real xanthan images. From the best-fit of the autocorrelation function on the orientation angle, the longest rotational relaxation time was effectively determined. Rotational relaxation time increases with the medium solvent viscosity due to the slow movement of xanthan molecule. It is confirmed that there is a good agreement between experiments and Brownian dynamics simulations on the relaxation patterns of xanthan chain.