Objective To determine the effect of circadian rhythm of blood pressure on cognitive function in the very old patients with hypertension. Methods A total of 96 senile hypertensive patients (older than 80 years old) admitted in our department from June 2013 to August 2014 were recruited in this study. According to the results of Mini-Mental State Examination (MMSE), the patients were divided into normal cognitive function group (control group, n＝61) and mild cognitive impairment group (MCI group, n＝35). The general conditions, complications and ambulatory blood pressure parameters were compared between the 2 groups. Results There was no significant difference in 24-hour systolic blood pressure (24hSBP) and 24-hour diastolic blood pressure (24hDBP) between the 2 groups [(131.14±12.61) vs (131.00±13.17)mmHg, and (65.74±7.35) vs (64.08±8.39)mmHg, P＞0.05). No difference was found either in day SBP (dSBP), day DBP (dDBP), night SBP (nSBP) and night DBP (nDBP) between the 2 groups (P＞0.05). However, morning surge in blood pressure was significantly higher in MCI group than in control group [(8.69±12.64) vs (4.08±12.78)mmHg, P＜0.05]. Conclusion Higher morning blood pressure is associated with cognitive impairment in the very old patients with hypertension.