The X-ray stress constants of five crystalline planes of beryllium were measured by using a cantilever beam loading method on X2001 stress analyzer. The results show that the X-ray stress constant of each plane in beryllium is different, so the X-ray stress constant corresponding to a diffraction plane must be adopted in order to acquire the exact stress value on X-ray stress analysis. Because of the rather large penetrate depth of X-rays into beryllium, that effect on X-ray stress constant measurement must be taken into account.