Objectives : The purpose of this study was to assess the effect of Taegeuk acupuncture on reducing mental stress by analyzing heart rate variability in Soyangin. Methods : Eight Soyangin-diagnosed women participated in this study. They were randomly divided into group A and group B. Each participant went through 3 sessions every week with 1 week of washout period in between each session. HRV was measured three times at every session; at baseline, after administering mentally stressful circumstances and after applying either one of simple rest, Soyangin Taegeuk acupuncture or Soeumin Taegeuk acupuncture. This study was designed as a crossover clinical trial. After same initial simple resting session for both groups at week 1, acupuncture for group A were executed in the order of Soyangin Taegeuk acupuncture and Soeumin Taegeuk acupuncture at week 2 and 3 respectively, with acupuncture for group B conducted in reverse order. Results : Simple rest and Soeumin Taegeuk acupuncture did not show the significant changes in response to LF(norm) and HF(norm) after stress stimuli. Soyangin Taegeuk acupuncture did, however, significantly decrease LF(norm) and increase HF(norm). Soyangin Taegeuk acupuncture, compared to Soeumin Taegeuk acupuncture, significantly stabilized autonomic nervous system. Conclusions : This study suggests that Soyangin Taegeuk acupuncture might be an effective means of stabilizing mental stress-induced imbalance of autonomic nervous system for Soyangin.