The normal value of intraocular pressure is usually 10mmHg-21mmHg (1.33kPa-2.80kPa). IOP is the pressure inside the eye, referred to as intraocular pressure. It is the equalized pressure exerted by the contents of the eye on the wall of the eye. In normal people, the IOP is stabilized within a certain range to maintain the normal shape of the eye and to keep the refractive media interfaces in good refractive condition. The normal range of IOP in normal people is 10mmHg-21mmHg (1.33kPa-2.80kPa), with a mean value of 15.8mmHg (1mmHg≈0.133kPa) and a standard deviation of 2.6mmHg. From the statistical concept, it also defines normal IOP at 10-21mmHg (mean±2×standard deviation), but in practice, IOP in normal people is not normally distributed. Therefore, IOP >21 mmHg cannot be mechanically considered as a pathological value. Other factors affecting IOP include body position, which is 2-3 mmHg higher when lying than when standing, pressure on the eyeball, and forceful blinking, which can temporarily increase IOP. Clinically, although some patients’ IOP has exceeded the upper limit of statistical normal, long-term follow-up does not show optic nerve and visual field damage, which is called hyperopia. Some patients with IOP within the normal range have typical glaucomatous optic nerve atrophy and visual field defects, which is called normal IOP glaucoma. This shows that high IOP is not always glaucoma, and normal IOP does not exclude glaucoma. Normal IOP is 10mmHg-21mmHg (1.33kPa-2.80kPa) and the difference in pressure between the two eyes is <4mmHg-5mmHg (0.53kPa-0.67kPa).