There is a need to differentiate between patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD) from normal-aged controls (NC) in the field of clinical drug discovery. In this study, we developed a tactile angle discrimination system and examined whether the ability to discriminate tactile angle differed between patients with MCI and AD and the NC group. Thirty-seven subjects were divided into three groups: NC individuals (n - 14); MCI patients (n - 10); and probable AD patients (n = 13). All subjects were asked to differentiate the relative sizes of the reference angle (60 degrees) and one of eight comparison angles by passive touch. The accuracy of angle discrimination was measured and the discrimination threshold was calculated. We discovered that there were significant differences in the angle discrimination thresholds of AD patients compared to the NC group. Interestingly, we also found that ability to discriminate tactile angle of MCI patients were significantly lower than that of the NC group. This is the first study to report that patients with MCI and AD have substantial performance deficits in tactile angle discrimination compared to the NC individuals. This finding may provide a monitor and therapeutic approach in AD diagnosis and treatment.