Heat-not-burn (HNB) tobacco smoking has spread throughout the market. While it is suggested that HNB tobacco smoking reduces the vascular endothelial function and is associated with a high risk of developing cardiovascular disease. The antioxidant of vitamin C may attenuate the unfavorable effects of HNB tobacco smoking. In the present study, we examined the effect of oral vitamin C ingestion on the flow-mediated dilation (FMD) at the brachial artery and oxidative stress markers in patients before and after transient HNB tobacco smoking. Twelve healthy adult males underwent high-resolution ultrasonography of the brachial artery and evaluations of reactive oxygen metabolites (d-ROMs) and the biological antioxidant potential (BAP) before and after a single session of HNB smoking. FMD was used to examine the endothelial function and the oxidative stress and antioxidant status were determined by using a FRES4 analyzer. In this randomized, crossover, controlled trial, measurements were performed on 2 different days 20 min after the oral administration of 1000 mg of ascorbic acid (VC trial) or a placebo (P trial). Although the FMD values decreased after a single HNB smoking session in both trials, the VC trial showed significantly higher values than the P trial at 60 and 120 min after smoking. Whereas the FMD values 120 min after smoking in the P trial were lower compared to the Pre values, there was no difference in the VC trial. These results suggested that the ingestion of vitamin C might suppress the decrease in the endothelial function caused by a single HNB smoking.