Isothermal iodine adsorption behaviors of seven natural and regenerated cellulose fibers observed through the indirect I$_2$/KI/Na$_2$SO$_4$procedure were analyzed with Fowler-Guggenheim equation. The cellulose-iodine interaction energy was found to be -4.2 KCal/mole, regardless of the origin of cellulose fibers. The adsorbed iodine-iodine interaction energies of natural and regenerated cellulose fibers were -0.9 and -0.7Kcal/mole, respectively.