To assess the effect of acupuncture on the human, 17 healthy volunteers participated in a double-blind randomized crossover clinical trial using real-needle and no-needle seals. In this study, by using systolic blood pressure variability (SBP) and heart rate variability (HR), not only coherence function (K2) between SBP and HR but also two causal coherence functions: K2SBP→HR from SBP to HR and K2 HR→SBP from HR to SBP were introduced to divide causal linearity of the cardiovascular system regarded as a closed-loop system. K2 represents total linearity of the system. K2SBP→HR and K2 HR→SBP correspond to the baroreflex system and the mechanical hemodynamics, respectively. The results showed that K 2SBp→HR over the low frequency band (0.04-0.15Hz) underwent a lot of change in real-needle acupuncture but did not in no-needle acupuncture. And there was difference in K2HR→SBP over the high frequency band (0.15-0.45Hz) immediately after the stimulation of acupuncture. These results have never been obtained from conventional methods dealing with a cardiovascular system as an open-loop system.