The presence of inherent discontinuities within a rock mass poses significant influence on its shear strength-deformation characteristics. Therefore, it is important to study the rock joint performance within the laboratory for the safe and economical design of underground structures (such as mine roadways) in jointed rock mass, stability analysis of jointed rock slopes and foundation design on a fractured rock mass. To study the rock joint mechanics and principles governing its shear behaviour, this research is focused on the behaviour of natural rough rock joint within the laboratory under Constant Normal Load (CNL) and Constant Normal Stiffness (CNS) boundary condition using servo-controlled direct shear apparatus at the University of Wollongong, Australia. It was observed from the experimental results that the CNS testing procedure truly simulates the shearing mechanism of an actual field unlike CNL, and the shear strength evaluated rock joint is underestimated under the CNL boundary condition. Also, the shear strength envelope under CNS exhibits non-linearity in contrast with the bilinear strength envelope under CNL boundary condition.