Primary open angle glaucoma(POAG)is one of the eye diseases which can lead to blindness. The early symptom of POAG is unconspicuous, however, it will result in irreversible optic never damage and visual field defect with the disease progress. As an index for diagnosing and evaluating the therapeutic effect of POAG, intraocular pressure(IOP)is simple and important. Clinically, although some treated patients have a target IOP at clinic time, the optic never damage still get worse. Researches indicate that higher nocturnal IOP and 24h IOP variation and lower nocturnal ocular perfusion pressure(OPP)may be the reasons for this phenomenon. The following essay will give an overview of POAG,IOP and OPP according to the relevant literatures so that we can have a better understanding.