By using of heart rate variability (HRV) analysis, we can access the status of the autonomic nervous system (ANS) that can control and adjust heart rate at rest and exercise. HRV analysis is a simple, noninvasive method. It can measure sympathetic nervous activity and parasympathetic nervous activity. Recent studies have shown that athletes' aerobic exercise ability increases obviously after taking a long-term, planned exercise training. Also the activity of the parasympathetic nerve turns to prevail over the activity of the sympathetic nerve at the same time. In other words, the reason that athletes have higher HRV is because they have higher parasympathetic nerve activity. Hence HRV has a significant meaning no matter in analysis of exercise physiology and exercise science.