NMDA neurotransmission plays an important role in learning and memory. NMDA receptors were found to decrease in the frontal lobe and hippocampus of mild cognitive impairment. This study is a randomized, double-blind, placebo-controlled drug trial. All subjects will be allocated randomly to 2 groups: (1) NMDA-enhancer: DAOIB group (starting dose: 250-500 mg/d); (2) placebo group. The study period is 24 weeks. The investigators hypothesize that DAOIB may yield better efficacy than placebo for cognitive function in patients with mild cognitive impairment.