Objective To observe the characteristics of cognitive impairment in patients with primary hypothyroidism. Methods A total of 90 primary hypothyroidism patients untreated with thyroid hormone were selected.All the 90 patients were divided into subclinical hypothyroidism group, mild or moderate hypothyroidism group and serious hypothyroidism group, and 30 patients in each group.The other 30 healthy volunteers were selected as controls. Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA) were used for the testing their orientation, immediate memory, attention and calculation, delayed recalling, linguistic competence, visual space and execution, naming ability and abstracting power. One-way analysis of variance was performed to determine significant differences among four groups. Results The MMSE scores in subclinical hypothyroidism group , mild or moderate hypothyroidism group and serious hypothyroidism group((27.53±2.16), (26.90±1.88)and (24.80±2.10)respectively) were lower than those of the control (28.23±1.33). The MoCA scores of the above hypothyroidism groups((23.57±3.33), (21.60±2.81) and (20.53±3.03)respectively) were also lower than that of the control (26.63±2.31)(P<0.05). Except for orientation and immediate memory, statistical significances of the other cognitive function were existed between hypothyroidism groups and the healthy controls (P<0.05). With the increase in severity of hypothyroidism, the abnormality of attention, calculation, linguistic competence, visual space and executive ability, naming ability and abstracting power were appearing gradually in hypothyroidism groups(P<0.05), and the scores were low(P<0.05). Conclusion Defects of attention and calculation, delayed recalling, linguistic competence, visual space and execution, naming ability and abstracting power are existed in primary hypothyroidism patients. Key words: Primary hypothyroidism; Cognitive impairment; Subclinical hypothyroidism