Objective To analyze the characteristics of diurnal intraocular pressure (IOP) curves in normal tension glaucoma (NTG) patients and primary open angle glaucoma (POAG) patients by measuring IOP over 24 hours. Methods This was a prospective case control study. Subjects diagnosed as POAG and NTG were enrolled from April 2006 to April 2009 in the Shanghai Beizhan Hospital and Eye & ENT Hospital of Fudan University. A total of 131 NTG patients and 102 POAG patients were included in this study. All the subjects accepted 24-hour IOP measurements by a non-contact tonometer every two hours starting at 8:00 am in a sitting position. The IOP between 0:00 to 6:00 am were also measured in a sitting position immediately after the subject was awakened. The main observation indices included an IOP fluctuating curve, average IOP, peak time and IOP, valley time and IOP and the IOP fluctuating value. Differences in these indices between the two groups were analyzed by a independent samples t test and chi-square test. Results The average IOP in the NTG group were (14.2±2.1)mmHg for the two eyes, and in the POAG group were (19.9±3.5)mmHg. The peak IOP in the NTG group were (17.0±2.4)mmHg for the two eyes, and in the POAG group were (24.7±4.3)mmHg. The valley IOP in the NTG group were (11.8±2.2)mmHg for the two eyes, and in the POAG group were (16.5±3.1)mmHg. IOP fluctuations in the NTG group were (5.2±1.8)mmHg for the two eyes, and in the POAG group were (8.2±3.1)mmHg. All differences between the two groups were statistically significant (t=-14.52,-16.44,-13.16,-8.90, P＜0.01 for all). Notably, 63.3% of the peak IOP in the NTG and 73.5% of the peak IOP in the POAG occurred outside working hours, especially during the period from 0:00 to 6:00 am with 51.5% peak IOP in the NTG group and 64.7% in the POAG group. Differences between the two groups were statistically significant (x2=8.150, P=0.017). Conclusion The fluctuation curves of IOP in NTG and POAG patients are similar over 24 hours. These properties are not only guidelines for establishing individual treatment, but also for evaluating the therapeutic effects and treatment adjustments during follow-up.