Type 2 diabetes mellitus (T2DM) is associated with increased risk of cognitive impairment. Individuals with T2DMhave 1.5-2.5 folds increased risk of dementia compared with those without diabetes. Even in prediabetic stage, there is a gradual progression of subtle cognitive impairments involving memory, attention, processing speed, and executive functioning. Previous magnetic resonance imaging (MRI) studies demonstrated that patients with T2DM exhibited increased global brain atrophy and vascular lesions, with reduced cerebral blood flow than those without diabetes. Decreased regional spontaneous neural activation and disrupted functional connectivity in brains regions involving global cognitive processing were observed in T2DM patients in functional MRI (fMRI) studies. In addition, olfactory behavior dysfunction, characterized by increased odor thresholds and impaired odor discrimination and recognition, is considered as one of the earliest manifestations of neurodegenerative diseases and a potential preclinical biomarker of future cognitive decline. Noteworthy, lower scores in olfactory behavior test were also shown in T2DM patients compared with nondiabetics subjects and were significantly correlated with poor cognitive performance. This study firstly evaluated olfactory behavior and olfactory-induced brain activation in T2DM patients with normal cognitive status, providing new insights into the role of olfactory circuit alterations in the development of cognitive decline. Significantly altered olfactory-induced brain activations and disrupted functional connectivity were shown before both brain structural changes and clinical cognitive decrements in patients with T2DM. Such functional alterations in the olfactory network could probably constitute a potential marker for early cognitive decline in T2DM.