A direct shear equipment was modified for testing rock joints with suction control. Modification includes a vapour forced convection circuit connected to the air chamber of shear cell for controlling relative humidity during testing. The main features of the device are described and test program procedures are presented. In addition the results of tests conducted under different suction, net normal stress and asperity roughness angle conditions are presented and analyzed. Finally, new expressions of resistance parameters of hyperbolic failure envelope are proposed incorporating the effect of suction and asperity roughness angle.