Although many studies have shown no significant change in global cognitive function after subthalamic brain stimulation (STN DBS) in patients with Parkinson disease (PD) and have concluded that STN DBS is generally safe from a cognitive standpoint, some studies have reported a decline in global cognitive function after STN DBS. Interestingly, in some studies, the decline in cognitive function appears to be greater during the initial short period after surgery (within 6 or 12 months after surgery) than the decline thereafter. To this end, we examined whether the rate of change in global cognitive function during the initial 6 months after STN DBS was different from the mean 6-month change that occurred between 6 and 36 months after surgery. Thirty-six PD patients who underwent bilateral STN DBS and were followed for more than 3 years were included. Change in Mini-Mental Status Examination (MMSE) score during the first 6 months after surgery was compared with the 6-month MMSE score change between 6 and 36 months after surgery. Mean MMSE change during the first 6 months after surgery was significantly greater than the mean 6-month MMSE change between 6 to 36 months after surgery. The levodopa equivalent daily dose at baseline and the score for Stroop Color-word test at baseline were significantly associated with the decline in MMSE score during the first 6 months after surgery. Our result showed that decline in global cognitive function was faster in the first 6 months after surgery, compared with that after 6 months.