Objective: Low birth weight (LBW) was confirmed as a risk of high blood pressure in later stages of life (1, 2). Although, a number of studies have shown that Mongolian people have a higher prevalence of hypertension compared to other ethnic groups (3), including the Japanese, the underlying mechanism is still unclear. The purpose of this study was to identify whether there are differences in the relationship of LBW and the risk of hypertension between Japanese and Mongolian. Method: We performed electrocardiographic measurements in 13 Japanese and 16 Mongolian healthy volunteers aged between 19 and 33 years. We measured blood pressure (BP) and heart rate variability (HRV) at resting condition and postural change for both Japanese as well as Mongolian volunteers with LBW and normal birth weight (NBW) [LBW: 2,065 ± 179.7 g; NBW: 3,044 ± 141.2 g for Japanese vs LBW: 2,500 ± 131 g; NBW: 3,360 ± 67 g for Mongolian], respectively. All the data are expressed as mean ± SE. Student t-test and simple linear regression analysis were performed to compare the two variables. One-way repeated measures ANOVA were also performed to identify the influences of postural change. Results: In the resting condition, heart rate (HR), BP, high frequency (HF), the ratio of low frequency to high frequency (LF/HF) showed no differences between LBW group and NBW group in the Japanese volunteers. On the other hand, HR, LF/HF were inversely related with birth weight (HR: r = -0.518, p < 0.05; LF/HF: r = -0.647, p < 0.01) in Mongolian volunteers. Japanese LBW group showed a significant increase in HR (p < 0.05) from supine to sitting position. However, there was no significant increase in HR for Mongolian LBW group. NBW group in both Japanese and Mongolian showed significant decrease in HF after postural change. While the Japanese LBW group showed a significant increase (p <0.05) in LF/HF immediately after sitting position, the Mongolian counterparts did not show any such increase. Conclusion: Although, there was no relationship between birth weight and BP in the Japanese young adults who participated in the present study, the Mongolians showed an inverse correlation of birth weight with resting HR and sympathetic activity. In Mongolian LBW group, no significant increase in HR, sympathetic activity and decrease in parasympathetic nervous activity were observed after postural change. These results support that LBW is associated with increased sympathetic activity and reduced baroreflex sensitivity in young adults. We conclude that Mongolian young adults have a higher risk of hypertension compared to their Japanese counterparts.