The time constant, number of optokinetic after nystagmus(OKAN), and initial slow phase eye velocity(first two second) of OKAN were measured as parameters in normal adults(n=7) and unilateral vestibular functional loss patients(n=8). In the unilateral group when the optokinetic stimuli produced optokinetic nystagmus(OKN) with fast phase to the affected ear, OKAN was markedly diminished or absent. The OKAN response that the opposite stimulus produced was similar to that of normal adults. Among parameters the time constant of OKAN was best for side difference in patients with loss of unilateral vestibular function. But the number of OKAN and initial slow phase eye movement showed asymmetry in normal and unilateral vestibular function loss patients. So OKAN test give us a good information for side difference in patients with unilateral vestibular function loss.