Peripheral autonomic function is impaired in diabetic polyneuropathy. However, it is difficult to evaluate it due to the lack of non-invasive quantitative assessment. We aimed to establish a novel index to evaluate vascular autonomic function using reactive hyperemia peripheral arterial tonometry (RH-PAT), a widely performed endothelial function test. Sixty-five subjects were enrolled, including healthy subjects, cases with sympathetic nerve blockers, and diabetic patients. RH-PAT was performed with 5-min blood flow occlusion in unilateral arm. We calculated the reduction ratio of the post-occlusion pulse amplitude to the baseline in the non-occluded arm (RPN), with 1-min sliding window. In healthy subjects, RPN gradually increased with time-dependent manner. However, this phenomenon was eliminated in cases with sympathetic nerve blockers. Plasma concentration of norepinephrine was measured before and after the blood flow occlusion, which showed a significant increase. We then compared RPNs with the change in heart rate variability (HRV) parameters. RPN calculated at 5 min after the reperfusion had the highest correlation with the change in sympathetic HRV parameter, and thus, we named sympathetic hypoemia index (SHI). Finally, we studied the relationship between SHI and diabetes. SHI was significantly lower in diabetic patients than matched controls. SHI, a novel index derived from RH-PAT, represented the peripheral sympathetic activity. SHI may be useful for assessing the vascular autonomic activity in diabetic patients.