Rock joints of 30—50 m away from the earth surface are under lower confining pressure which is about 1 MPa. So it is of actual significance to investigate the flow properties of rock joints under lower pressure. Two hypotheses for rock joint deformation under lower pressure are put forward

of which one is that rock joint is linear elastic

and another is that aperture decrement is much smaller than the initial mechanical aperture. With these hypotheses

a semi-empirical theoretical formula for rock joint under lower normal stresses is developed using Barton’s equation. Experimental tests of marble joints are performed; and the results are consistent with the formula. It is shown that the hypotheses are reasonable

and that hydraulic conductivity linearly decreases with normal stress increasing. Furthermore

an indirect method for measuring initial mechanical aperture is given.