The step velocity of the tetragonal hen egg-white lysozyme (110)crystal plane toward the [001] direction could be controlled and measured accurately on a nanometer scale using a specially designed sealed sample vessel on an atomic force microscope. The effects of lysozyme and NaCl concentration on the step velocity were also examined. The step velocity was a non-linear function of supersaturation (sigma). The heights of steps and 2D islands corresponded to a bimolecular layer and the dominant growth unit was estimated as a tetramer. Crystal growth apparently stopped when the supersaturation was reduced to less than 0.69.