Objective : The purpose of this study was to analyze in detail the relationship between outcome and time course of effect in medically refractory primary cervical dystonia (CD) with phasic type that was treated by bilateral globus pallidus internus (Gpi) deep brain stimulation (DBS). Methods : Six patients underwent bilateral implantation of DBS into the Gpi under the guide of microelectrode recording and were followed for $18.7{pm}11.1$ months. The mean duration of the CD was $5.8{pm}3.4$ years. The mean age at time of surgery was $54.2{pm}10.2$ years. Patients were evaluated with the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) and relief scale using patient self-reporting. Results : The TWSTRS total scores improved by 64.5%, 65.5%, 75.8%, and 76.0% at 3, 6, 12 months, and at the last available follow-up after surgery, respectively. Statistically significant improvements in the TWSTRS scores were observed 3 months after surgery (p=0.028) with gradual improvement up to 12 months after surgery, thereafter, the improvement was sustained. However, there was no statistically significant difference between the scores at 3 and 12 months. Subjective improvement reported averaged $81.7{pm}6.8%$ at last follow-up. Mild dysarthria, the most frequent adverse event, occurred in 3 patients. Conclusions : Our results show that the bilateral Gpi-DBS can offer a significant therapeutic effect from 3 months postoperatively in patients with primary CD with phasic type, without significant side effects.