Abstract: In this research, the hydraulic characteristics of a horizontal disc, which is assumed as a rock joint, have been studied analytically and numerically with reference to radial flow occurred along a typical water pressure test that is also known as Lugeon test. The hydraulic characteristics such as Lugeon value, i.e., flowrate to hydrostatic pressure ratio, the joint hydraulic aperture, inertia conditions, turbulence and laminar conditions and the effect of relative roughness were investigated by use of well-known hydraulic relations coincided by numerical analysis. The numerical results were regarded as a verifier to evaluate the accuracy of empirical and analytical relations. The comparison between analytical and numerical results was based on Lu value which is recorded as the main result of Lugeon test. The results show that the simplest equation (the Darcy equation of radial flow) demonstrates the closer estimation to numerical one, in comparison with other analytical and empirical equations. Also, the Lugeon test must be considered as a single-well test and it could just identify a combination of some hydraulic parameters. A practical procedure is finally presented to calculate the possible hydraulic characteristics of tested section in the borehole according to Water Pressure test.