Summary Acupuncture is a peripheral stimulation technique that has been used to treat pain disorders for thousands of years and is gaining popularity in Western countries. However, clinical trials evaluating the analgesic efficacy of acupuncture treatment have found only a modest specific effect. Transcranial direct current stimulation (tDCS) is a safe, low cost, and clinically applicable neuromodulation tool that can modulate cortical excitability. Accumulating evidence suggest that tDCS (at the primary motor gyrus) may be a promising treatment for chronic pain. Further studies showed that both acupuncture and tDCS may work through the descending pain modulation system, and pilot studies have suggested that combining the two may produce a greater therapeutic effect than each alone. In this study, we will investigate the brain mechanism of the synergistic effects of combining acupuncture and tDCS treatments. Specifically, chronic low back pain (cLBP) patients will be recruited and randomized to one of four groups (30 per group, one month of treatment): 1) verum acupuncture + real tDCS, 2) sham acupuncture + real tDCS, 3) verum acupuncture + sham tDCS, and 4) sham acupuncture + sham tDCS. We will study 1) the longitudinal (one-month) effects of different treatments as indicated by changes in resting state functional connectivity (rsFC), cerebral blood flow (CBF), clinical outcomes of low back pain, and quantitative sensory test (QST) and 2) the association between these changes. Both acupuncture and tDCS are promising therapeutic methods for chronic pain. Elucidating the brain mechanisms of acupuncture, tDCS, and the synergistic effects of the two will facilitate the development of new pain management methods and promote improvements in pain care.